Can I use CSS Box Alignment ?

As a member of the Igalia’s team implementing the CSS Grid Layout feature for Blink and WebKit rendering engines, I’m very proud of what we’ve achieved from our collaboration with Bloomberg. I think Grid is a very interesting feature for the Web Platform and we still can’t see all its potential.

One of my main assignments on this project is to implement the CSS Box Alignment spec for Grid. It’s obvious that alignment is an important feature for many cases in web development, but I consider it a key for a layout model like the one Grid provides.

We recently announced that the patch implementing the self-baseline alignment landed in Blink. This was the last alignment functionality pending to implement, so now we can consider that the spec is complete for Grid. However, implementing a feature like CSS Box Alignment has an additional complexity in the form of interoperability issues.

Interoperability is always a challenge when implementing any new specification, but I think it’s specially problematic for a feature like this for several reasons:

  • The feature applies to several layout models.
  • The CSS Flexible Box specification already defined some of the CSS properties and values.
  • Once a new layout model implements the new specification, Flexbox is forced to follow it as well.

I admit that the editors of this new specification document made a huge effort to keep backward compatibility with the Flexbox spec (which caused not so few implementation challenges). However, the current Flexbox implementation of the CSS properties and values that both specs have in common would become a Partial Implementation regarding the new spec.

Recently Florian Rivoal found out that this partial implementation of the CSS Box Alignment feature prevents the use of cascade or @support for providing customized fallbacks for the unimplemented Alignment properties.

What does Partial Implementation actually mean ?

As anybody can imagine, implementing a fancy web feature takes a considerable amount of time. During this period, the feature passes through several phases with different exposure to the end users. It’s precisely due to the importance of end user’s feedback that these new web features are shipped under experimental flags. This workflow is specially useful no only for browser devs but for the spec editors as well.

For this reason, the W3C CSS Working Group defines a general policy to manage Partial Implementations, which can be summarized as follows:

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported property values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.

This policy is added to every spec as part of its Conformance appendix, so it is in the case of the CSS Box Alignment specification document. However, the interpretation of the Partial Implementation policy is far from trivial, specially for a feature like CSS Box Alignment. The most restrictive interpretation would imply the following facts:

  • Any new CSS property of the new spec should be declared invalid until is supported by all the layout models it applies to.
  • Any of the already existent CSS properties with new values defined in the new spec should be declared invalid until all these new values are implemented in all the layout models such property applies to.
  • Browsers shouldn’t ship (without experimental flags) any CSS property or value until it’s implemented in all the layout model it applies to.

When we discussed about this at Igalia we applied a less restrictive interpretation, based on the assumption that the spec actually defined several features which could be implemented and shipped independently, obviously avoiding any browsers interoperability issues. As it’s been always in the nature of the specification, keeping backward compatibility with Flexbox implementations has been a must, since its spec already defines some of the CSS properties now present in the new spec.

The issue filed by Florian was discussed during the Tokyo F2F Apr 19-21 2017 meeting, where it was agreed to add a new section in the CSS Box Alignment spec to clarify how implementors of this feature should manage Partial Implementations:

Since it is expected that support for the features in this module will be deployed in stages corresponding to the various layout models affected, it is hereby clarified that the rules for partial implementations that require treating as invalid any unsupported feature apply to any alignment keyword which is not supported across all layout modules to which it applies for layout models in which the implementation supports the property in general.

The new text added makes the Partial Implementation policy less restrictive and, even it contradicts our interpretation of independent alignment features per layout model, it affects only to models which already implement any of the CSS properties defined in the new spec. In this case, only Flexbox has to be updated to implement the new values defined for its alignment related CSS properties: align-content, justify-content and align-self.

Analysis of the implementation and shipment status

Before thinking on how to address the Partial Implementation issues, I decided to analyze what’s the status of the CSS Box Alignment feature in the different browsers. If you are interested in the full analysis, it’s available here. The following table shows the implementation status of the new spec in the Safary, Chrome and Firefox browsers, using a color code like unimplemented, only grid or both (flex and grid):

If you can try out some examples of these Partial Implementation issues, just try flexbox vs grid cases with some of these alignment values: align-items: center, align-self: left; align-content: start or justify-content: end.

The 3 major browsers analyzed have shipped most, if not all, the CSS Box Alignment spec implemented for CSS Grid Layout (since Chrome 57, Safari 10.1, Firefox 52). Firefox is the browser which implemented and shipped a wider support for CSS Flexible Box.

We can extract the following conclusions:

  • The 3 browsers analyzed have shipped Partial Implementations of the CSS Box Alignment specification, although Firefox is almost complete.
  • The 3 browsers have shipped a Grid feature that supports completely the new CSS Box Alignment spec, although Safari still misses the self-baseline values.
  • The 3 implementations of the new CSS Box Alignment specification are backward compatible with the CSS Flexible Box specification, even though it implements for some properties a lower level of the spec (e.g. self-baseline keywords)

Work in progress

Although we are still evaluating the problem together with the Blink and WebKit communities, at Igalia we are already working on improving the situation. We all agree on the damage to the Web Platform that these Partial Implementation issues are causing, as Florian pointed out initially, so that’s a good starting point. There are bug reports on both WebKit and Blink and we are already providing patches for some of them.

We are still discussing about the best approach, but our bet would be to request an intent-to-implement-and-ship for a CSS Box Alignment (for flexbox layout) feature. This approach fits naturally in our initial plans of implementing several independent features from the alignment specification. It seems that it’s what Firefox is doing, which already announced the implementation of CSS Box Alignment (for block layout)

Thanks to Bloomberg for sponsoring this work, as part of the efforts that Igalia has been doing all these years pursuing a better and more open web.

Igalia & Bloomberg logos

Web Engines Hackfest 2016

Last week I attended the Web Engines Hackfest 2016, hosted by Igalia at the HQ premises in A Coruña. For those still unaware, it’s a unconference like event focused on pure hacking and technical discussions about the main Web Engines supporting the Web Platform.

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This year there were a very interesting group of hackers, representing most of the main web engines, like Mozilla’s Gecko and Servo, Google’s Blink, Apple’s WebKit and Igalia’s WebKitGTK+.

Hacking log

This year I was totally focused on hacking Blink web engine to solve some of the most complex issues of the CSS Grid Layout feature. I’d like to start giving thanks to Google to join the hackfest sending Christian Biesinger, specially thanks to him because of the long flight to attend. It was a pleasure to work work with him on-site and have the opportunity to discuss these complex issues face to face.

Day 1

During the weeks previous to the hackfest I’ve been working on fixing the bug 628565, reported several months ago but hitting me quite much recently. It’s a very ugly issue, since it shows an unpredictable behavior of Grid layout logic, so I decided that I might fix it once for all. I managed to provide 2 different approaches, which can be analyzed in the following code review issues: Issue 2361373002 and Issue 2333583002. Basically, the root cause of both issues is the same; grid’s container intrinsic size is not computed correctly when there are grid items with orthogonal flows.

There are 2 fundamental concepts that are key for understanding this problem:

  • Intrinsic size computation must be done before layout.
  • Orthogonal flow boxes need to be laid out in order to compute min-content contribution to their container’s intrinsic width.

So, we had as single bug to fix for solving two different problems: unpredictable grid layout logic and incorrect grid’s intrinsic size computation. The first problem is the one reported in bug 628565. I’ll try to describe it here briefly, but further details can be obtained from the bug report. Let’s consider the following code:

<style>
   body { overflow: hidden; }
</style>
<div style="width: fit-content; display: grid; grid-template-rows: 50px; border: 5px solid; font: 25px/1 Ahem;">
   <div style="writing-mode: vertical-lr; color: magenta; background: cyan;">XX X</div>
</div>

The following pictures illustrate the issue when loading the code above with Google Chrome 55.0.2859.0 (Official Build) dev (64-bit) Revision 3f63c614e8c4501b1bfa3f608e32a9d12618b0a0-refs/heads/master@{#418117} under Linux operating system:

Chrome BEFORE resizing
chrome-before-resizing

Chrome AFTER resizing
chrome-after-resizing

Christian and I analyzed carefully Blink’s layout code and how it deals with orthogonal boxes. It’s worth mentioning that there are important issues with orthogonal flows in almost every web engine, but Blink has made lately some improvements on this regard. See how a very basic example works in Blink, Gecko and WebKit engines:

<div style="float: left; border: 5px solid; font: font: 25px/1 Ahem;"></div>
  <div style="writing-mode: vertical-lr; color: magenta; background: cyan;">XX X</div>
</div>

orthogonal-flow-different-engines

Blink has implemented a kind of pre-layout logic which is executed for any orthogonal flow box even before doing the actual layout, when box’s intrinsic size computation takes place. This solution allows using a more precise info about an orthogonal box’s height when computing its container’s intrinsic width; otherwise zero width would be assumed, as it’s the case of WebKit and Gecko engines. However, this approach leads to an incorrect behavior when using Grid Layout as I’ll explain later.

My first approach to solve these two issues was to update grid container’s intrinsic size after its orthogonal children were laid out. Hence, we could use the actual size of the children to compute their container’s intrinsic size. However, this violates the first rule of the two assertions defined before: no layout should be done for intrinsic size computation.

Layout Breakout Session

During the evening we held the Layout Breakout Session, where we had a nice discussion about the future of Layout in the different web engines. Christian Biesinger, one of the members of the Google’s Blink Layout team, talked about the new LayoutNG project; an experiment to implement a new Layout from scratch, cleaning up quite old code paths and solving some problems that were not possible to address with the current legacy code. This new LayoutNG idea is related to the new Layout API and the Houdini project, a new mechanism for defining new layout models without the requirement of a native support inside the browser. We had also some discussions about the current state of the Flexible Box specification in Blink and how WebKit’s implementation is quite abandoned and unmaintained nowadays.

In addition, we discussed about the current state of CSS Grid Layout implementation in the different engines. The implementation is almost complete in most of the main engines. Thanks to the collaboration between Igalia and Bloomberg we can confirm that WebKit and Blink’s implementations are almost completed. We have been evaluating Mozilla’s Gecko implementation of Grid and we verified it’s in a similar status. We talked about the recent news from TPAC, which Manuel Rego attended, about the CSS Grid Layout specification becoming Candidate Recommendation. Because of all these reasons, we have agreed with Christian that it’d be good to send the Blink intent-to-ship request as soon as possible; in case it’s accepted, it could be enabled by default in the next Chrome release.

Day 2

The day started with a meeting with Christian for evaluating the different approaches we implemented so far. We have discarded the ones requiring updating intrinsic size. We also decided to avoid solving the issue during the pre-layout of orthogonal items. This approach would have been the one with less impact on performance for grid layout and it would also solve the incorrect intrinsic size issue, however it would add penalties for cases not using grid at all.

Finally, Christian and I agreed on solving first the unpredictable behavior of grid layout logic. We would skip the issue of incorrect intrinsic size, overall because we think the Grid Layout specification is contradictory on this regard. For what is worth, I’ve created a new issue for the CSSWG in the W3C’s github. Even though we should wait for the issue to get clarified, we have already some possible approaches for getting what seems a more natural result for grid’s intrinsic size. The following example could help to understand the problem:

intrinsic-sizes-with-orthogonal

Both test cases were loaded using the same Chrome version commented before. They clearly show that neither min-content or max-content sizes are applied correctly to the grid container. The reason of this weird behavior is how content-sized tracks are handled by the Grid Tracks sizing algorithm in case of rendering grid items with orthogonal flow. From the last draft specification:

Then, if the min-content contribution of any grid items have changed based on the row sizes calculated in step 2, steps 1 and 2 are repeated with the new min-content contribution and max-content contribution (once only).

This, with the fact that orthogonal boxes pre-layout is performed before the tracks have been defined, causes that grid’s container intrinsic size is computed incorrectly. This problem is explained in detail in the W3C’s github issue mentioned before. So if anybody is interested on additional details or, even better, willing to participate in the ongoing discussion, just follow the link above.

Day 3

In addition to the orthogonal flow issues, Baseline Alignment was the other hot topic of my work during the hackfest. Baseline Alignment is the only feature still missing to complete the implementation of the CSS Box Alignment specification for Grid Layout. There is a preliminary approach in this code review issue, but it’s still not complete. The problem is that as it’s stated in the Alignment spec, Baseline Alignment may affect grid’s container intrinsic size:

When specified for align-self/justify-self, these values trigger baseline self-alignment, shifting the entire box within its container, which may affect the sizing of its container.

This fact implies that I should integrate Baseline offset computation inside the Grid Tracks sizing algorithm. Christian and I have been analyzing the sizing algorithm and we designed a possible approach, quite similar to what Flexbox implements in its layout logic.

Finally, we met again with Christian to discuss about the Minimum Implied size for Grid issues. Manuel had the chance to discuss it with the Grid spec editors at TPAC, but it seems there are still unresolved issues. There are an ongoing discussion at W3C’s github about this problem, you can get the details in issues 283 and 523. Christian suggested that he could add some feedback to the discussion, so we can clarify it as soon as possible. This is an important issue that may affect browser interoperability.

The hackfest

The Web Engines Hackfest is an event to share experiences between hackers of different web engines and brainstorming about the future of the Web Platform. This year there were hackers representing most of the main web engines, including Google’s Blink, Mozilla’s Gecko and Servo, Apple’s WebKit and Igalia’s WebKitGTK+.

webengines-1

There were scheduled talks from hackers of each engine so everybody could get an idea of their current state and future plans. In addition, some breakout sessions were scheduled during the kick-off session driven by my colleague Martin Robinson. We embraced everybody to propose new breakout sessions on the fly, every time an ongoing discussion needed a deeper debate or analysis.

ctrbqf6wcaatsvk-jpglarge

I could attend most of the talks and breakout sessions, so I’ll give now my impressions about them. All the talks were recorded (will be available soon) and slides are available in the wiki, so I recommend to watch them if you haven’t already.

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The first speaker in the schedule was Jack Moffitt (Mozilla) “Servo: Today & Tomorrow”. He gave a great overview of the progress they made on the new Servo rendering engine, emphasizing the multi-thread support and showing some performance metrics for different engine’s components, CSS parsing, scripting, layout, … He remarked the technical advantages of using Rust on the development of this new engine.

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During Day 1 I attended two breakout sessions. The first one about the WebKitGTK+ engine was mainly focused on the recently published roadmap and specially about the new Threaded Compositor enabled by default. There was an interesting discussion about graphics support in WebKitGTK+ between the Collabora developers and Andrey Fedorov (Huawei).

29851562800_e6b17e78bd_o

The other breakout session I could attend during Day 1 was the one about the Servo engine. There was a nice discussion between Mozilla developers and Christian Biesinger about how both engines handle rendering in a different way. Servo hackers explained with more detail Servos’s threading model and its implication for layout.

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Day 2 started with the awesome talk by Behdad Esfahbod (Google) “HarfBuzz, the First Ten Years”. He gave an overview of the evolution of the HarfBuff library along the last years, including the past experiences in the Web Engines Hackfest and former events under previous name WebKitGTK+ Hackfest. Clearly, rendering fonts is a huge technical challenge. It was also awesome to have Behdad here to work closely with my colleague Fred Wang on the fonts support for MathML.

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I attended the MathML breakout session, where the hot topic was Fred’s prototype implemented for the Blink engine. The result of this experiment was really promising, so we started to discuss its chances of getting back in Chrome, even behind an experimental flag. Both Christian and Behdad expressed doubts about that being possible nowadays. They suggested that it may be feasible to implement it through Houdini and the new Blink’s Layout API. We have confirmed that the refactoring we have done for WebKit applies perfectly in Blink, since both share a substantial portion of the layout codebase. In addition, after our work for removing the Flexbox dependency and further code clean up, we can be confident on the MathML independence in the layout logic. After some discussion, we have agreed on continue our experiments in Blink, independently of its chances to be integrated in Blink in the future, since I don’t think the Houdini approach makes sense for MathML, at least now.

Later on Youenn Fabler (Apple) gave a talk about the Fetch API, “Fetching Bytes and Words on the Web”. He described the implementation of this specification in WebKit and its relation with the Streams API. The development is still ongoing and there are still quite much work to do.

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I missed the talks of Day 3, so hopefully I’ll watch them as soon as the videos are available. It’s specially interesting the talk about WPE (aka WebKit For Wayland) , by my colleague Žan Doberšek. I’ve spent the day trying to get the most of having Christian Biesinger at our premises. As I commented before, we discussed about the Grid Layout’s Implied Minimum Size issue, one of the most complex problems we’ll have to solve in the short term.

Performance analysis of Grid Layout

Now that we have a quite complete implementation of CSS Grid Layout specification it’s time to take care of performance analysis and optimizations. In this essay, which is the first of a series of posts about performance, I’ll first introduce briefly how to use Blink (Chrome) and WebKit (Safari) performance analysis tools, some of the most interesting cases I’ve seen during my work on the implementation of this spec and, finally, a basic case to compare Flexbox and Grid layout models, which I’d like to evolve and analyze further in the coming months.

Performance analysis tools

Both WebKit and Blink projects provide several useful and easy to use scrips (python) to run a set of test cases and take different measurements and early analysis. They were written before the fork, that’s why related documentation can be found at WebKit’s track, but both engines still uses them, for the time being.

Tools/Scripts/run-perf-tests
Tools/Scripts/webkitpy/performance_tests/

There are a wide set of performance tests under PerformanceTest folder, at Blink’s/WebKit’s root directory, but even though both engines share a substantial number of tests, there are some differences.

(blink’s root directory) $ ls PerformanceTests/
Bindings BlinkGC Canvas CSS DOM Dromaeo Events inspector Layout Mutation OWNERS Parser resources ShadowDOM Skipped SunSpider SVG XMLHttpRequest XSSAuditor

Chromium project has introduced a new performance tool, called Telemetry, which in addition of running the above mentioned tests, it’s designed to execute more complex cases like running specific PageSets or doing benchmarking to compare results with a preset recording (WebPageRelay). It’s also possible to send patches to performance try bots, directly from gclient or git (depot_tools) command line. There are quite much information available in the following links:

Regarding profiling tools, it’s possible both in Webkit and Blink to use the –profiler option when running the performance tests so we can collect profiling data. However, while WebKit recommends perf for linux, Google’s Blink engine provides some alternatives.

CSS Grid Layout performance tests and current status

While implementing a new browser feature is not easy to measure performance while code evolves so much and quickly and, what it’s worst, be aware of regressions introduced by new logic. When the feature’s syntax changes or there are missing or incomplete functionality, it’s not always possible to establish a well defined baseline for performance. It’s also a though decision to determine which use cases we might care about; obviously the faster the better, but adding performance optimizations usually complicates code, it may affect its robustness and it could lead to unexpected, and even worst, hard to find bugs.

At the time of this writing, we had 3 basic performance tests:

Why we have selected those uses cases to measure and keep track of performance regression ? First of all, note that auto-sizing one of the most expensive branches inside the grid track sizing algorithm, so we are really interested on both, improving it and keeping track of regressions on this code path.

body {
    display: grid;
    grid-template-rows: repeat(100, auto);
    grid-template-columns: repeat(20, auto);
}
.gridItem {
    height: 200px;
    width: 200px;
}

On the other hand, fixed-sized is the easiest/fastest path of the algorithm, so besides the importance of avoiding regressions (when possible), it’s also a good case to compare with auto-sized.

body {
    display: grid;
    grid-template-rows: repeat(100, 200px);
    grid-template-columns: repeat(20, 200px);
}
.gridItem {
    height: 200px;
    width: 200px;
}

Finally, a stretching use cases was added because it’s the default alignment value for grid items and the two test cases already described use fixed size items, hence no stretch (even though items fill the whole grid cell area). Given that I implemented CSS Box Alignment support for grid I was conscious of how expensive the stretching logic is, so I considered it an important use case to analyze and optimize as much as possible. Actually, I’ve already introduced several optimizations because the early implementation was quite slow, around 40% slower than using any other basic alignment (start, end, center). We will talk more about this later when we analyze a case to compare Flexbox and Grid performance in layout.

body {
    display: grid;
    grid-template-rows: repeat(100, 200px);
    grid-template-columns: repeat(20, 200px);
}
.gridItem {
    height: auto;
    width: auto;
}

The basic HTML body of these 3 tests is quite simple because we want to analyze performance of very specific parts of the Grid Layout logic, in order to detect regressions in sensible code paths. We’d like to have eventually some real use cases to analyze and create many more performance tests, but chrome performance platform it’s definitively not the place to do so. The following graphs show performance evolution during 2015 for the 3 tests we have defined so far.

grid-performance-overview

Note that yellow trace shows data taken from a reference build, so we can discount temporary glitches on the machine running the performance tests of target build, which are shown in the blue trace; this reference trace is also useful to detect invalid regression alerts.

Why performance is so different for these cases ?

The 3 tests we have for Grid Layout use runs/second values as a way to measure performance; this is the preferred method for both WebKit and Blink engines because we can detect regressions with relatively small tests. It’s possible, though, to do other kind of measurements. Looking at the graphs above we can extract the following data:

  • auto-sized grid: around 650 runs/sec
  • fixed-sized grid: around 1400 runs/sec
  • fixed-sized stretched grid: around 1250 runs/sec

Before analyzing possible causes of performance drop for each case, I’ve defined some additional tests to stress even more these 3 cases, so we can realize how grid size affect to the obtained results. I defined 20 tests for these cases, each one with different grid items; from 10×10 up to 200×200 grids. I run those tests in my own laptop, so let’s take the absolute numbers of each case with a grain of salt; although differences between each of these 3 scenarios should be coherent. The table below shows some numeric results of this experiment.

grid-fixed-VS-auto-VS-stretch

First of all, recall that these 3 tests produce the same web visualization, consisting of grids with NxN items of 100px each one. The only difference is the grid layout strategy used to produce such result: auto-sizing, fixed-sizing and stretching. So now, focusing on previous table’s data we can evaluate the cost, in terms of layout performance, of using auto-sized tracks for defining the grid (which may be the only solution for certain cases). Performance drop is even growing with the number of grid items, but we can conclude that it’s stabilized around 60%. On the other hand stretching is also slower but, unlike auto-sized, in this case performance drop does not show a high dependency of grid size, more or less constant around 15%.

grid-performance-graphs-2

Impact of auto-sized tracks in layout performance

Basically, the track sizing algorithm can be described in the following 4 steps:

  • 1- Initialize per Grid track variables.
  • 2- Resolve content-based TrackSizingFunctions.
  • 3- Grow all Grid tracks in GridTracks from their baseSize up to their growthLimit value until freeSpace is exhausted.
  • 4- Grow all Grid tracks having a fraction as the MaxTrackSizingFunction.

These steps will be executed twice, first cycle for determining column tracks’s size and another cycle to set row tracks’s size which it may depend on grid’s width. When using just fixed-sized tracks in the very simple case we are testing, the only computation required to determine grid’s size is completing step 1 and determining free available space based on the specified fixed-size values of each track.

// 1. Initialize per Grid track variables.
for (size_t i = 0; i < tracks.size(); ++i) {
    GridTrack& track = tracks[i];
    GridTrackSize trackSize = gridTrackSize(direction, i);
    const GridLength& minTrackBreadth = trackSize.minTrackBreadth();
    const GridLength& maxTrackBreadth = trackSize.maxTrackBreadth();
 
    track.setBaseSize(computeUsedBreadthOfMinLength(direction, minTrackBreadth));
    track.setGrowthLimit(computeUsedBreadthOfMaxLength(direction, maxTrackBreadth, track.baseSize()));
 
    if (trackSize.isContentSized())
        sizingData.contentSizedTracksIndex.append(i);
    if (trackSize.maxTrackBreadth().isFlex())
        flexibleSizedTracksIndex.append(i);
}
for (const auto& track: tracks) {
    freeSpace -= track.baseSize();
}

Focusing now on the auto-sized scenario, we will have the overhead of resolving content-sized functions for all the grid items.

// 2. Resolve content-based TrackSizingFunctions.
if (!sizingData.contentSizedTracksIndex.isEmpty())
    resolveContentBasedTrackSizingFunctions(direction, sizingData);

I didn’t add source code of resolveContentBasedTrackSizingFunctions because it’s quite complex, but basically it implies a cost proportional to the number of grid tracks (minimum of 2x), in order to determine minContent and maxContent values for each grid item. It might imply additional computation overhead when using spanning items; it would require to sort them based on their spanning value and iterate over them again to resolve their content-sized functions.

Some issues may be interesting to analyze in the future:

  • How much each content-sized track costs ?
  • What is the impact on performance of using flexible-sized tracks ? Would it be the worst case scenario ? Considering it will require to follow the four steps of track sizing algorithm, it likely will.
  • Which are the performance implications of using spanning items ?

Why stretching is so performance drain ?

This is an interesting issue, given that stretch is the default value for both Grid and Flexbox items. Actually, it’s the root cause of why Grid beats Flexbox in terms of layout performance for the cases when stretch alignment is used. As I’ll explain later, Flexbox doesn’t have the optimizations I’ve implemented for Grid Layout.

Stretching logic takes place during the grid container layout operations, after all tracks have their size precisely determined and we have properly computed all grid track’s positions relatively to the grid container. It happens before the alignment logic is executed because stretching may imply changing some grid item’s size, hence they will be marked for layout (if they wasn’t already).

Obviously, stretching only takes place when the corresponding Self Alignment properties (align-self, justify-self) have either auto or stretch as value, but there are other conditions that must be fulfilled to trigger this operation:

  • box’s computed width/height (as appropriate to the axis) is auto.
  • neither of its margins (in the appropriate axis) are auto
  • still respecting the constraints imposed by min-height/min-width/max-height/max-width

In that scenario, stretching logic implies the following operations:

LayoutUnit stretchedLogicalHeight = availableAlignmentSpaceForChildBeforeStretching(gridAreaBreadthForChild, child);
LayoutUnit desiredLogicalHeight = child.constrainLogicalHeightByMinMax(stretchedLogicalHeight, -1);
 
bool childNeedsRelayout = desiredLogicalHeight != child.logicalHeight();
if (childNeedsRelayout || !child.hasOverrideLogicalContentHeight())
    child.setOverrideLogicalContentHeight(desiredLogicalHeight - child.borderAndPaddingLogicalHeight());
if (childNeedsRelayout) {
    child.setLogicalHeight(0);
    child.setNeedsLayout();
}
 
LayoutUnit LayoutGrid::availableAlignmentSpaceForChildBeforeStretching(LayoutUnit gridAreaBreadthForChild, const LayoutBox& child) const
{
    LayoutUnit childMarginLogicalHeight = marginLogicalHeightForChild(child);
 
    // Because we want to avoid multiple layouts, stretching logic might be performed before
    // children are laid out, so we can't use the child cached values. Hence, we need to
    // compute margins in order to determine the available height before stretching.
    if (childMarginLogicalHeight == 0)
        childMarginLogicalHeight = computeMarginLogicalHeightForChild(child);
 
    return gridAreaBreadthForChild - childMarginLogicalHeight;
}

In addition to the extra layout required for changing grid item’s size, computing the available space for stretching adds an additional overhead, overall if we have to compute grid item’s margins because some layout operations are still incomplete.

Given that grid container relies on generic block’s layout operations to determine the stretched width, this specific logic is only executed for determining the stretched height. Hence performance drop is alleviated, compared with the auto-sized tracks scenario.

Grid VS Flexbox layout performance

One of the main goals of CSS Grid Layout specification is to complement Flexbox layout model for 2 dimensions. It’s expectable that creating grid designs with Flexbox will be more inefficient than using a layout model specifically designed for these cases, not only regarding CSS syntax, but also regarding layout performance.

However, I think it’s interesting to measure Grid Layout performance in 1-dimensional cases, usually managed using Flexbox, so we can have comparable scenarios to evaluate both models. In this post I’ll start with such cases, using a very simple one in this occasion. I’d like to get more complex examples in future posts, the ones more usual in Flexbox based designs.

So, let’s consider the following simple test case:

<div class="className">
   <div class="i1">Item 1</div> 
   <div class="i2">Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.</div>
   <div class="i3">Item 3 longer</div>
</div>

I evaluated the simple HTML example above with both Flexbox and Grid layouts to measure performance. I used a CPU profiler to figure out where the bottlenecks are for each model, trying to explain where differences came from. So, I defined 2 CSS classes for each layout model, as follows:

.flex {
    background-color: silver;
    display: flex;
    height: 100px;
    align-items: start;
}
.grid {
    background-color: silver;
    display: grid;
    grid-template-columns: 100px 1fr auto;
    grid-template-rows: 100px;
    align-items: start;
    justify-items: start;
}
.i1 { 
    background-color: cyan;
    flex-basis: 100px; 
}
.i2 { 
    background-color: magenta;
    flex: 1; 
}
.i3 { 
    background-color: yellow; 
}

Given that there is not concept of row in Flexbox, I evaluated performance of 100 up to 2000 grid or flex containers, creating 20 tests to be run inside the chrome performance framework, described at the beginning of this post. You can check out resources and a script to generate them at our github examples repo.

flexVSgrid

When comparing both layout models targeting layout times, we see clearly that Grid Layout beats Flexbox using the default values for CSS properties controlling layout itself and alignment, which is stretch for these containers. As it was explained before, the stretching logic adds an important computation overhead, which as we can see now in the numeric table above, has more weight for Flexbox than Grid.

Looking at the plot about differences in layout time, we see that for the default case, Grid performance improvement is stabilized around 7%. However, when we avoid the stretching logic, for instance by using any other alignment value, layout performance it’s considerable worse than Flexbox, for this test case, around 15% slower. This is something sensible, as this test case is the idea for Flexbox, while a bit artificial for Grid; using a single Grid with N rows improves performance considerably, getting much better numbers than Flexbox, but we will see these cases in future analysis.

Grid layout better results for the default case (stretch) are explained because I implemented several optimizations for Grid. Probably Flexbox should do the same, as it’s the default value and it could affect many sites using this layout model in their designs.

Thanks to Bloomberg for sponsoring this work, as part of the efforts that Igalia has been doing all these years pursuing a better and more open web.

Igalia & Bloomberg logos

Distributing tracks along Grid Layout container

In my last post I introduced the concept of Content Distribution alignment and how it affects Grid Layout implementation. At that time, it was possible to use all the <content-position> values to select grid tracks position inside a grid container, moving them across the available space. However, it wasn’t until recently that users can distribute grid tracks along such available space, literally adding gaps in between or even stretching them.

In this post I’ll describe how each <content-distribution> value affect tracks in a Grid Layout, their position and size, using different grid structures (eg. number of tracks, span).

Let’s start analyzing the new Content Distribution alignment syntax defined in the CSS Box Alignment specification:

auto | <baseline-position> | <content-distribution> || [ <overflow-position>? && <content-position> ]

In case of a <content-distribution> value can’t be applied, its associated fallback <content-distribution> value should be used instead. However, this CSS syntax allow users to specify a preferred fallback value:

If both a <content-distribution> and <content-position> are given, the <content-position> provides an explicit fallback alignment.

Before going into each value, I think it’s a good idea to refresh the concepts of alignment container and alignment subject and how they apply in the context of Grid Layout:

The alignment container is the grid container’s content box. The alignment subjects are the grid tracks.

The different <content-distribution> values that can be used for align-content and justify-content CSS properties are defined as follows:

  • space-betweenThe alignment subjects are evenly distributed in the alignment container. Default fallback: start.
  • space-aroundThe alignment subjects are evenly distributed in the alignment container, with a half-size space on either end. Default fallback: center.
  • space-evenlyThe alignment subjects are evenly distributed in the alignment container, with a full-size space on either end. Default fallback: center.
  • stretchAny auto-sized alignment subjects have their size increased equally (not proportionally) so that the combined size exactly fills the alignment container. Default fallback: start.

Picture below describes how these values would behave depending on the number of grid tracks; for simplicity I only use justify-content property, so tracks are distributed along the inline (row) axis. In next examples we will see how both properties work together using more complex grid definitions.

content-distribution-1
Effect of different Content Distribution values on Grid Layout. Click on the Image to evaluate the behavior when using different number of tracks.

Previous examples were defined with grid items filling grid areas of just 1×1 tracks, which makes distribution pretty simple and easier to predict. But thanks to the flexibility of Grid Layout syntax we can define irregular grids, for instance, using the grid-template-areas property like in the next example.

align-content-and span-4
Basic example of how to apply the different values and its effect on irregular grid design.

Since Content Distribution alignment considers grid tracks as the alignment subject, distributing tracks along the available space may have the consequence of modifying the dimensions of grid-areas defined by more than one track. The following picture shows the result of the code above and provides and excellent example of how powerful is the Content Alignment effect on a Grid Layout.

These use cases can be obtained from Igalia’s Grid Layout examples repository, so anybody can play with different grid designs and alignment values combinations. They are also available at our codepen repository.

Grid Layout behind the scene

Now I’d like to explain a bit what I had to implement in the browser’s webcore to get these new features done; just some small pieces of source code, the ones I considered better, to get an idea of what implementing new behavior in browsers implies.

As you might already know because of my previous posts, CSS Box Alignment specification was born to generalize Flexbox’s alignment behavior so that it can be used for grid and even regular blocks. Several new properties were added, like justify-items and justify-self, and CSS syntax has changed considerably. Specially noteworthy how Content Distribution alignment properties have changed from their initial Flexbox definition. They now support complex values like ‘space-between true’, ‘space-around start’, or even ‘stretch center safe’. This makes possible to express more info than using the previous simple keyword form, although it requires a new CSS parsing logic in Browsers.

More complex CSS parsing

Since both align-content and justify-content properties accept multiple optional keywords I needed to re-implement completely their parsing logic. I’m happy to announce that it recently landed WebKit’s trunk too, so now both web engines support the new CSS syntax.

Due to the complex values defined for theses CSS properties, a new CSSValue derived class was defined to hold all the Content Alignment data, named as CSSContentDistributionValue. This data is then converted to something meaningful for the style logic using the StyleBuilderConverter class. This is the preferred method in both WebKit and Blink engines, which it just needs to be declared in the CSSPropertyNames.in and CSSProperties.in template files, respectively.

align-content initial=initialContentAlignment, converter=convertContentAlignmentData
justify-content initial=initialContentAlignment, converter=convertContentAlignmentData

The StyleBuildConverter logic is pretty simple thanks to these 2 new data structures, as it can be appreciated in the following excerpt of source code:

StyleContentAlignmentData StyleBuilderConverter::convertContentAlignmentData(StyleResolverState&amp;, CSSValue* value)
{
    StyleContentAlignmentData alignmentData = ComputedStyle::initialContentAlignment();
    CSSContentDistributionValue* contentValue = toCSSContentDistributionValue(value);
    if (contentValue->distribution()->getValueID() != CSSValueInvalid)
        alignmentData.setDistribution(*contentValue->;distribution());
    if (contentValue->position()->getValueID() != CSSValueInvalid)
        alignmentData.setPosition(*contentValue->;position());
    if (contentValue->overflow()->getValueID() != CSSValueInvalid)
        alignmentData.setOverflow(*contentValue->overflow());
    return alignmentData;
}

The StyleContentAlignmentData class was defined to simplify how we manage these complex values, so that we can handle properties as they had an atomic value. This approach allows a more efficient and robust way of detecting and managing style changes in these properties.

New Layout operations

Once this new CSS syntax is correctly parsed and a LayoutStyle instance generated according to user defined CSS style rules, I needed to modify Flexbox’s layout code for adapting it to the new data structures, ensuring browser backward compatibility and passing all the Layout and Unit tests. I implemented from scratch this logic for Grid Layout so I had the opportunity to introduce several performance optimizations to avoid unnecessary layouts and repaints. This area is pretty interesting and I’ll talk about it soon in a new post.

One interesting aspect of Content Distribution alignment is that it might take part in the track sizing algorithm. As it was explained in my previous post about Self Alignment, stretch value increases alignment subject’s size to fill its alignment container’s available space. This is also the case of Content Alignment, but considering tracks as alignment subject. However, there is another case not so obvious where <content-distribution> values may influence in track sizing resolution, or perhaps better said, grid area sizing.

Let’s consider this example of grid where there are certain areas using more than one track:

grid-template-areas: "a a b"
                     "c d b"
grid-auto-columns: 20px;
grid-auto-rows: 40px;
width: 150px;
height: 300px;

The example above defines a grid with 3 column tracks of 20px and 2 row tracks of 40px, which would be laid out as it’s shown in the following diagram:

content-distribution-spans
Grid Layout with areas filing more than one track. Click on the picture to evaluate the effect of each value on the grid area size.

This fact has interesting implementation implications due to the fact that in certain cases, in order to determine grid item’s logical height we need its logical width to be resolved first. Track sizing algorithm uses children grid area size to determine grid cell’s logical height, hence given that alignment logic needs track sizes have been already resolved, it may imply a re-layout of the grid items which size could be affected by the used content-distribution value. The following source code shows how I handle this scenario:

LayoutUnit LayoutGrid::gridAreaBreadthForChild(const LayoutBox& child, GridTrackSizingDirection direction, const Vector& tracks) const
{
    const GridCoordinate& coordinate = cachedGridCoordinate(child);
    const GridSpan& span = (direction == ForColumns) ? coordinate.columns : coordinate.rows;
    const Vector& trackPositions = (direction == ForColumns) ? m_columnPositions : m_rowPositions;
    if (span.resolvedFinalPosition.toInt() < trackPositions.size()) {
        LayoutUnit startOftrack = trackPositions[span.resolvedInitialPosition.toInt()];
        LayoutUnit endOfTrack = trackPositions[span.resolvedFinalPosition.toInt()];
        return endOfTrack - startOftrack + tracks[span.resolvedFinalPosition.toInt()].baseSize();
    }
    LayoutUnit gridAreaBreadth = 0;
    for (GridSpan::iterator trackPosition = span.begin(); trackPosition != span.end(); ++trackPosition)
        gridAreaBreadth += tracks[trackPosition.toInt()].baseSize();
 
    return gridAreaBreadth;

The code above will return different results, in the cases mentioned before, depending on whether it’s run during track sizing alignment or after applying the alignment logic. This will likely make needed a new layout of the whole grid, or at least, the affected grid items, which it likely has a negative impact on performance.

Current status and next steps

I’d like to finish this post with a snapshot of current situation and challenges for the next months, as I’ve been regularly doing in my last posts.

Unlike last reports, this time I’ve got good news regarding reduction of implementation gaps between the two web engines we are focusing our efforts on, WebKit and Blink. The following table describes current situation:

alignment-status

The table above indicates that several milestones were reached since the last report, although there are still some pending issues:

  • I’ve completed the implementation in WebKit of the parsing logic for the new Box Alignment properties: align-items and align-self.
  • As a side effect, I’ve also upgraded the ones already present because of Flexbox to the latest CSS3 Box Alignment specification.
  • WebKit has now full support for Default and Self Alignment fro Grid Layout, including also overflow handling
  • Blink has now full support for Content Distribution alignment, which missed <content-distrbuton> values.
  • WebKit’s Grid Layout implementation still misses support for Content Distribution alignment.
  • Baseline Alignment is still missing in both web engines

In addition to the above mentioned pending issues, our roadmap include the following tasks as part of my todo list for the next months:

  • Even though there s support for different writing-modes and flow directions, there are still some issues with orthogonal flows. I’ve got already some promising patches but they still have to be reviewed by Blink and WebKit engineers.
  • Optimizations of style and repaint invalidations triggered by changes on the alignment properties. As commented before, this is a very interesting topic involving, which I’ll elaborate further in next posts.
  • Performance analysis of relevant Grid Layout use cases, which hopefully will lead to optimizations proposals.

All this work and many other contributions to Grid Layout for WebKit and Blink web engines are the result of the collaboration between Bloomberg and Igalia to implement this W3C specification.

Igalia & Bloomberg logos

Content Distribution in CSS Grid Layout

It’s been a while since Igalia and Bloomberg started to implement the Box Alignment specification for the CSS Grid Layout model. Some weeks ago we accomplished an important milestone of our roadmap landing in Blink trunk the last patches implementing the Content Distribution properties: align-content and justify-content.

Quoting the CSS Box Alignment document, the content distribution properties are defined as follows:

Aligns the contents of the box as a whole along the box’s inline/row/main axis.
The alignment container is the grid container’s content box. The alignment subjects are the grid tracks.

The CSS syntax of these recently added properties gives an idea of how powerful and flexible they are for grid layout definitions, allowing every possible alignment values combination:

auto | <baseline-position> | <content-distribution> || [ <overflow-position>? && <content-position> ]

It’s worth mentioning that Baseline Alignment is still not implemented, as well as the <content-distribution> values for Distribution Alignment. However, in the latter case I’ve got already a quite promising draft implementation which, eventually, has been very useful to activate a discussion inside the W3C community to allow these alignment values for grid. In previous versions of the specification it was stated that all <content-distribution> values should use their <content-position> fallback values for grid containers. I’m glad that such decision was finally made, because I think that <content-distribution> values are really useful for defining fancier grid layouts. I’ll talk about this soon in a new post, as I consider it deserves a detailed description with the proper examples.

Last, but not least, as it happens with Self Alignment it allows using overflow keywords to define how we want to handle grid’s content overflow. It works in the same way for Content Distribution as we’ll see later with some examples.

Aligning the grid

When there is available space in the grid container block, it’s always useful to have a way to control how we want to use such space and how we want our grid to behave on it. It might happen that container’s size changes (fullscreen mode) or we could have to deal with a content sized grid modifying its content’s size. There are quite many possibilities, so I’ll leave this issue for user/designer’s imagination and I’ll focus on very simple examples to illustrate the concept.

For now, let’s consider this case to understand what you can do with the different <content-alignment> values in a grid layout.

.grid {
    grid: 50px 50px / 100px 100px;
    position: relative;
    width: 200px;
    height: 300px;
}
.fixedSize {
    width: 20px;
    height: 40px;
}
<div class="grid">
   <div class="fixedSize" style="grid-column: 1; grid-row: 1; background: violet;"></div>
   <div style="grid-column: 1; grid-row: 2; background: yellow;"></div>
   <div style="grid-column: 2; grid-row: 1; background: green;"></div>
   <div class="fixedSize" style="grid-column: 2; grid-row: 2; background: red;"></div>
</div>

We are defining a 2×2 grid with 50×100 pixels cells where we are going to place 4 items, one in each cell. Notice that items at (1,1) and (2,2) have a fixed size of 20×40 pixels, while the other 2 are auto-sized so they will be stretched to fill their corresponding grid cell (if you don’t know why, a reading of previous post might help). Also, bear in mind that both align-content and justify-content properties have start as the initial value for grids.

ContentAlignment

Controlling the grid overflow

When grid content’s size exceeds its container dimensions there is the risk of data loss. Some examples of this scenario are center or end alignment from the viewport’s edges; all the content overflowing the viewport’s area can’t be reached, hence we lose such data. In order to prevent this issue Box Alignment specification defines the safe overflow mode, which basically forces a start alignment value for the property handling the dimension where the overflow is detected.

Using the same CSS and HTML code in the example above, we can easily define cases where this data loss issue (red colored arrows) is clearly noticeable just modifying the height or width to cause top or left overflow respectively.

Content-Alignment-Overflow1

There are other situations where Content Alignment and Overflow interact in a different way, using margins, padding or/and borders and even combining different writing-modes and flow directions. The effect of the alignment values varies considerably depending on those factors but I think you have now a clear idea of how to use these new properties in a grid layout.

Current status and next steps

With the grid support for the align-content and justify-content CSS properties in Blink we’ve got most of the Box Alignment specification covered. As it was commented before, just Base Alignment is still pending to be implemented in Chromium browsers. I have to admit that there are also some bugs and wrong behavior using certain CSS combinations, specially regarding orthogonal flows, but we are working on it right now and I hope to integrate the patches soon in trunk.

For the time being, let’s consider the following table as the current implementation status of the Box Alignment specification for the Grid Layout model in WebKit (Safari/Epiphany) and Blink (Chrome/Chromium/Opera) based browsers:

align-grid-support-1

The lack of progress in the implementation of the Box Alignment specification in the WebKit web engine is disappointing. I’ve been stuck for quite a lot of time trying to upgrade the CSS properties to the last version of the spec, mainly due design and performance issues. I’ll discuss with the WebKit hackers the best approach to solve this issue so I can put the Grid Layout implementation at the same level than in Blink web engine.

Igalia and Bloomberg will continue working on the implementation of the CSS Grid Layout specification and among my short/mid term challenges are completing the Box Alignment support. These goals include the following tasks:

  • Fixing bugs and completing the orthogonal flows support.
  • Implementing the Base Alignment features
  • Completing the Content Distribution Alignment with the <content-distribution> values
  • Implementing the Box Alignment spec in WebKit
Igalia & Bloomberg logos

Igalia and Bloomberg working to build a better web platform

Box Alignment and Grid Layout (II)

Some time has passed since my first post about the Box Alignment spec implementation status for Blink and WebKit web engines. I’ll do an update in this post and, since the gap between both web engines has grown considerably (I’ll do my best to reduce it as soon as possible), I’ll remark the differences between both engines.

What’s new ?

The ‘stretch’ value is now implemented for align-{self, items} and justify-{self, items} CSS properties. This behavior is quite important because it’s the default for these four properties in Flexible Box and Grid layouts. According to the specification, the ‘stretch’ value is defined as follows:

If the width or height (as appropriate) of the alignment subject is auto, its used value is the length necessary to make the alignment subject’s outer size as close to the size of the alignment container as possible, while still respecting the constraints imposed by min-height/max-width/etc. Otherwise, this is equivalent to start.

When defining the alignment properties in a grid layout it’s very important to consider how we want to manage item’s overflow. It’s allowed to specify an overflow alignment value for both Content and Item Alignment definition, but so far it’s implemented only for the Item Alignment properties. The Overflow Alignment concept is defined in the specification as follows:

To help combat undesirable data loss, an overflow alignment mode can be explicitly specified. “True” alignment honors the specified alignment mode in overflow situations, even if it causes data loss, while “safe” alignment changes the alignment mode in overflow situations in an attempt to avoid data loss.

The ‘stretch’ value in Grid Layout

This value applies to the Self Alignment properties {align, justify}-self, and obviously their corresponding Default Alignment ones {align, justify}-items. For grids, these properties consider that the alignment container is the grid cell, while the alignment subject is the grid item’s margin box.

The Box Alignment specification states that Default Alignment ‘auto’ values should be resolved to ‘stretch’ in case of Grid containers; this value will be used as the resolved value for ‘auto’ Self Alignment values.

So by default, or when explicitly defined as ‘stretch’, the grid item’s margin box will be stretched to fill its grid cell breadth. Let’s see it with a basic example:

align-stretch
All the examples available at http://igalia.github.io/css-grid-layout/

This change affected only the layout codebase of the web engine, since the value was already introduced in the style parsing logic. The Grid Layout rendering logic uses an interesting abstraction to override the grid item’s block container. This abstraction allows us to use Grid Cells as block containers when computing the logical height and width.

Overflow Alignment in Grid Layout

The Overflow Alignment value used when defining a grid layout could be particularly useful, specially for fixed sized grids. The potential data lost may happen not only at the left and top box edges, but between adjoining grid cells. Overflow Alignment is defined via the ‘safe’ and ‘true’ keywords. They were already introduced in the Blink core’s style parsing logic as part of the CSS3 upgrade of the alignment properties (justify-self, align-self) used in the FlexBox implementation. The new CSS syntax is described by the following expression:

auto | stretch | <baseline-position> | [ <item-position> && <overflow-position>? ]

According to the current Box Alignment specification draft, the Overflow Alignment keywords have the following meaning:

  • safe: If the size of the alignment subject overflows the alignment container, the alignment subject is instead aligned as if the alignment mode were start.
  • true: Regardless of the relative sizes of the alignment subject and alignment container, the given alignment value is honored.

I’ll proceed now to show how Overflow Alignment is applied in the Grid Layout specification with an example:

align-overflow
All the examples available at http://igalia.github.io/css-grid-layout/

The new syntax to allow the Overflow Alignment keywords required to modify the style builder and parsing logic, as it was mentioned before. The alignment properties became a CSSValueList instance instead of simple keyword IDs; both Blink and WebKit provides Style Builder code generation directives (CSSPropertyNames.in) for simple properties, but this was not the case for these properties anymore.

The Style Builder is more complex now because it has to deal with the conditional overflow keyword, which can be specified before or after the <item-position> keyword. Blink provides a function template scheme for groups of properties sharing the same logic, which is the case of most of the CSS Box Alignment properties (align-self, align-items and justify-self; justify-items is slightly different so it needs custom functions). WebKit is currently defining the new style builder and it does not follow this approach yet, but I’d say it will, eventually, since it makes a lot of sense.

{% macro apply_alignment(property_id, alignment_type) %}
{% set property = properties[property_id] %}
{{declare_initial_function(property_id)}}
{
    state.style()->set{{alignment_type}}(RenderStyle::initial{{alignment_type}}());
    state.style()->set{{alignment_type}}OverflowAlignment(RenderStyle::initial{{alignment_type}}OverflowAlignment());
}
 
{{declare_inherit_function(property_id)}}
{
    state.style()->set{{alignment_type}}(state.parentStyle()->{{property.getter}}());
    state.style()->set{{alignment_type}}OverflowAlignment(state.parentStyle()->{{property.getter}}OverflowAlignment());
}
 
{{declare_value_function(property_id)}}
{
    CSSPrimitiveValue* primitiveValue = toCSSPrimitiveValue(value);
    if (Pair* pairValue = primitiveValue->getPairValue()) {
        state.style()->set{{alignment_type}}(*pairValue->first());
        state.style()->set{{alignment_type}}OverflowAlignment(*pairValue->second());
    } else {
        state.style()->set{{alignment_type}}(*primitiveValue);
    }
}
{% endmacro %}
{{apply_alignment('CSSPropertyJustifySelf', 'JustifySelf')}}
{{apply_alignment('CSSPropertyAlignItems', 'AlignItems')}}
{{apply_alignment('CSSPropertyAlignSelf', 'AlignSelf')}}

Even though style building and parsing is shared among all the layout models using the Box Alignment properties, like Flexible Box and Grid so far, Overflow Alignment is only supported so far by the CSS Grid Layout implementation. The Overflow Alignment logic affects how the grid items are positioned during the layout phase.

The Overflow Alignment keywords are also valid in the Content Distribution syntax, which I’m working on now with quite good progress. The first patches landed already in trunk, providing an implementation of the justify-content property in Grid Layout. I’ll talk about it soon, hopefully, once the implementation is completed and the discussion in the www-style mailing list conclude with some agreement regarding the Distributed Alignment for grids.

Current implementation status

The Box Alignment specification is quite complete now in Blink, unfortunately that’s not the case of WebKit. I’ll summarize now the current implementation status in browsers based on each web engine, which are basically Chrome/Chromium vs Safari; I’ll also try to outline the roadmap for the next weeks.

align-grid-support

The flex-start, and flex-end values are used only in Flexible Box layouts, so they don’t apply to this analysis of the Grid support of the Box Alignment spec. The Distributed Alignment values apply only to the Content Distribution properties (align-content and justify-content). Finally, ‘stretch‘ is a valid value for both, Positional and Distributed Alignment, so it’s not redundant but a different interpretation of the same value depending on the property it’s applied to.

Some conclusions we can extract from the table above:

  • Default and Self Alignment support is almost complete in Blink; only Baseline Alignment is pending to be implemented.
  • Content Distribution support for justify-content in Blink. Only <content-position> values are implemented, since current spec draft states that all the <content-distibution> values will fallback in Grid Layout; spec authors are still evaluating changes on this issue, though.
  • WebKit fails to provide CSS3 parsing for all the properties except justify-self, although there are some patches pending of review to improve this situation.
  • There is no Grid support at all in WebKit for any of the Box Alignment properties.
Igalia & Bloomberg logos

Igalia and Bloomberg working to build a better web platform

Web Engines Hackfest 2014

An awesome week is coming to the end and I’d like to thanks the sponsors for helping us to make possible that such an amazing group of hackers could work together to improve the web. We focused on some of the consolidated web engines but also about the most promising ones and, of course, hacking on them producing a good amount of patches.

15972643911_c197af2a23_z

The talks were great, and even better the breakout sessions about many hot topics for the future of the web engines.

15797464819_3eb5d51404_zBecause of the work I’ve been doing lately, I was specially involved in the CSS Features session, which among other things, it complemented the talk Sergio Villar gave us about the Grid Layout implementation we are doing at Igalia in collaboration with Bloomberg.  I introduced as well the work I’ve been doing on the implementation of the Box Alignment specification in both Blink and WebKit web engines; we evaluated how it would impact other layout models, like MathML, CSS Regions, CSS Flexible Box, to ease the logic of blocks and content alignment. We also discussed about the future of CSS regarding new layout models, which is a bit uncertain; there is actually an interesting discussion inside the W3C about this topic, so we will see how it evolves. We talked about graphics and CSS and the SVG specification  (the 2.0 version is being defined) which  will have an important role in the future, as I could personally notice during the last CSSConfEU conference in Berlin; it was also an important topic in other conferences along this year.

15789146369_b390b71cf8_zThis week was a nice opportunity to discuss with other web core hackers the issues I’ve found to properly implement the CSS Box Alignment specification in WebKit, see discussion in the bugzilla for details. We have concluded  that is not an easy problem that should be discussed in the mailing list, as it would imply assuming a performance overhead in CSS parsing. The ‘auto’ value defined by the spec for all the Box Alignment properties, to be resolved during the cascade depending on the type of elements, is affecting the current implementation of Flexible Box and MathML so we will have to find a solution.

I also produced a bunch of patches for WebKit to improve the way we were managing margins and float elements, properly identifying the elements creating a new formatting context and applying some refactoring to make the code clearer; these improvements fixed several issues in Grid Layout as well. Besides, borders, margin and padding was finally adapted in Grid Layout to the different writing-modes, which was a patch I’ve been working for some weeks already and had the opportunity to complete during this hackfest.

I think that’s all for now, hope to see you all in the next Web Engines Hackfest 2015.

sponsors

 

Box Alignment and Grid Layout

As some of my readers already know, Igalia and Bloomberg are collaborating in the implementation of the Grid Layout specification for the Blink/Chromium and WebKit web engines. As part of this assignment, I had the opportunity to review and contirbute to the implementaiton of another feature I consider quite useful for the web: CSS Box Alignment Module (level 3).

The Box Alignment specification was designed to generalize the behavior of boxes alignment within their containers, which is nowadays defined across multiple specifications. Several layout models are affected by this new specification: block, table, flex and grid. This post is about how it affects to the Grid Layout implementation.

I think is a good idea to begin my exposition with a brief introduction of some concepts related to alignment and CSS Writing Modes, which I consider quite relevant to understand the implications of this specification for the Grid Layout implementation and, more important, to realize about its potential.

Examples are mandatory when analyzing W3C specifications; personally, I can’t see all the angles and implications of a feature described in a specification without the proper examples, both visual and source code.

Finally, I’d like to conclude my article with a development angle describing some interesting implementation details and technical challenges I faced while working on both Blink and WebKit web engines. Also, which perhaps is more interesting, the ones I couldn’t solve yet and I’m still working on. As always comments and feedback are really welcome.

Introduction to Box Alignment and Writing-Modes

From the CSS Box Alignment specification:

features of CSS relating to the alignment of boxes within their containers in the various CSS box layout models: block layout, table layout, flex layout, and grid layout.

From the CSS Writing Modes specification:

CSS features to support for various international writing modes, such as left-to-right (e.g. Latin or Indic), right-to-left (e.g. Hebrew or Arabic), bidirectional (e.g. mixed Latin and Arabic) and vertical (e.g. Asian scripts).

In order to get a better understanding of alignment some abstract dimensional and directional terms should be explained and taken into account. I’m going to briefly describe some of them, the ones I consider more relevant for my exposition; a more detailed definition can be obtained from the Abstract Box Terminology section of the specification.

There are three sets of directional terms in CSS:

  • physical – Interpreted relative to the page, independent of writing mode. The physical directions are left, right, top, and bottom
  • flow-relative –  Interpreted relative to the flow of content. The flow-relative directions are start and end, or block-start, block-end, inline-start, and inline-end if the dimension is also ambiguous.
  • line-relative – Interpreted relative to the orientation of the line box. The line-relative directions are line-left, line-right, line-over, and line-under.

The abstract dimensions are defined below:

  • block dimension – The dimension perpendicular to the flow of text within a line, i.e. the vertical dimension in horizontal writing modes, and the horizontal dimension in vertical writing modes.
  • inline dimension – The dimension parallel to the flow of text within a line, i.e. the horizontal dimension in horizontal writing modes, and the vertical dimension in vertical writing modes.
  • block axis – The axis in the block dimension, i.e. the vertical axis in horizontal writing modes and the horizontal axis in vertical writing modes.
  • inline axis – The axis in the inline dimension, i.e. the horizontal axis in horizontal writing modes and the vertical axis in vertical writing modes.
  • extent or logical height – A measurement in the block dimension: refers to the physical height (vertical dimension) in horizontal writing modes, and to the physical width (horizontal dimension) in vertical writing modes.
  • measure or logical width – A measurement in the inline dimension: refers to the physical width (horizontal dimension) in horizontal writing modes, and to the physical height (vertical dimension) in vertical writing modes. (The term measure derives from its use in typography.)

Then, there are flow-relative and line-relative directions. For the time being, I’ll consider only flow-relative directions terms since they are more relevant for discussing alignment issues.

  • block-start – The side that comes earlier in the block progression, as determined by the writing-mode property: the physical top in horizontal-tb mode, the right in vertical-rl, and the left in vertical-lr.
  • block-end – The side opposite block-start.
  • inline-start – The side from which text of the inline base direction would start. For boxes with a used direction value of ltr, this means the line-left side. For boxes with a used direction value of rtl, this means the line-right side.
  • inline-end – The side opposite start.

writing-modes

So now that we have defined the box edges and flow direction concepts we can review how they are used when defining the alignment

properties and values inside a Grid Layout, which can be defined along two axes:

  • which dimension they apply to (inline vs. stacking)
  • whether they control the position of the box within its parent, or the box’s content within itself.

alignment-properties

Regarding the alignment values, there are two concepts that are important to understand:

  • alignment subject – The alignment subject is the thing or things being aligned by the property. For justify-self and align-self, the alignment subject is the margin box of the box the property is set on. For justify-content and align-content, the alignment subject is defined by the layout mode.
  • alignment container – The alignment container is the rectangle that the alignment subject is aligned within. This is defined by the layout mode, but is usually the alignment subject’s containing block.

Also, there are several kind of alignment behaviors:

  • Positional Alignment – specify a position for an alignment subject with respect to its alignment container.
  • Baseline Alignment – form of positional alignment that aligns multiple alignment subjects within a shared alignment context (such as cells within a row or column) by matching up their alignment baselines.
  • Distributed Alignment – used by justify-content and align-content to distribute the items in the alignment subject evenly between the start and end edges of the alignment container.
  • Overflow Alignment – when the alignment subject is larger than the alignment container, it will overflow. To help combat this problem, an overflow alignment mode can be explicitly specified.

At the time of this writing, only Positional Alignment is implemented so I’ll focus on those values in the rest of the article. I’m still working on implementing the specification, though, so there will be time to talk about the other values in future posts.

  • center – Centers the alignment subject within its alignment container.
  • start – Aligns the alignment subject to be flush with the alignment container’s start edge.
  • end – Aligns the alignment subject to be flush with the alignment container’s end edge.
  • self-start – Aligns the alignment subject to be flush with the edge of the alignment container corresponding to the alignment subject’s start side. If the writing modes of the alignment subject and the alignment container are orthogonal, this value computes to start.
  • self-end – Aligns the alignment subject to be flush with the edge of the alignment container corresponding to the alignment subject’s end side. If the writing modes of the alignment subject and the alignment container are orthogonal, this value computes to end.
  • left – Aligns the alignment subject to be flush with the alignment container’s line-left edge. If the property’s axis is not parallel with the inline axis, this value computes to start.
  • right – Aligns the alignment subject to be flush with the alignment container’s line-right edge. If the property’s axis is not parallel with the inline axis, this value computes to start.

So, after this introduction and with all these concepts in mind, it’s now time to get hands on the Grid Layout implementation of the Box Alignment specification. As it was commented before, I’ll try to use as many examples as possible.

Aligning items inside a Grid Layout

Before entering in details with source code and examples, I’d like to summarize most of the concepts described below with some pretty simple diagrams:

2×2 Grid Layout (LTR)

grid-alignment-ltr

2×2 Grid Layout (RTL)

grid-alignment-rtl

The diagram below illustrates how items are placed inside the grid using different writing modes:

grid-writing-modes

At this point, some real examples would help to understand how the CSS alignment properties work on Grid Layout and why they are so important to get all the potential behind this new layout model.

Let’s consider this basic stylesheet which will be used in the examples from now on:

<style>
  .grid {
      grid-auto-columns: 100px;
      grid-auto-rows: 200px;
      width: -webkit-fit-content;
      margin-bottom: 20px;
  }
   .item {
      width: 20px;
      height: 40px;
  }
   .content {
      width: 10px;
      height: 20px;
      background: white;
  }
   .verticalRL {
      -webkit-writing-mode: vertical-rl;
  }
   .verticalLR {
      -webkit-writing-mode: vertical-lr;
  }
   .horizontalBT {
      -webkit-writing-mode: horizontal-bt;
  }
   .directionRTL {
      direction: rtl;
  }
</style>

The item style will be used for the grid items, while the content will be the style of the elements to be placed inside each grid item. There are as well writing-mode related styles, which will be useful later to experiment with different flow and text directions.

In the first example we will center all the cells content so we can have a fully aligned grid, which is particularly interesting for many web applications.

<div class="grid" style="align-items: center; 
                         justify-items: center">
  <div class="cell row1-column1">
    <div class="item"></div>
  </div>
  <div class="cell row1-column2">
    <div class="item"></div>
  </div>
  <div class="cell row2-column1">
    <div class="item"></div>
  </div>
  <div class="cell row2-column2">
    <div class="item"></div>
  </div>
</div>
grid-alignment-example1

In the next example we will illustrate how to use all the Positional Alignment values so we can place nine items in the same grid cell.

 
<div class="grid">
  <div class="cell row1-column1"
     style="align-self: start; justify-self: start;">
    <div class="item"></div>
  </div>
  <div class="cell row1-column1"
     style="align-self: center; justify-self: start;">
    <div class="item"></div>
  </div>
  <div class="cell row1-column1"
     style="align-self: end; justify-self: start;">
    <div class="item"></div>
  </div>
  <div class="cell row1-column1"
     style="align-self: start; justify-self: center;">
    <div class="item"></div>
  </div>
  <div class="cell row1-column1"
     style="align-self: center; justify-self: center;">
    <div class="item"></div>
  </div>
  <div class="cell row1-column1"
     style="align-self: end; justify-self: center;">
    <div class="item"></div>
  </div>
  <div class="cell row1-column1"
     style="align-self: start; justify-self: end;">
    <div class="item"></div>
  </div>
  <div class="cell row1-column1"
     style="align-self: center; justify-self: end;">
    <div class="item"></div>
  </div>
  <div class="cell row1-column1"
     style="align-self: end; justify-self: end;">
    <div class="item"></div>
  </div>
</div>
grid-alignment-example2

Let’s start playing with inline and block-flow direction and see how it affects to the different Positional Alignment values. I’ll start with the inline direction, which affects only to the justify-xxx set of CSS properties.

<div class="grid" style="align-items: self-start; justify-items: self-start">
  <div class="cell row1-column1">
    <div class="item"></div>
  </div>
  <div class="cell row1-column2">
    <div class="item"></div>
  </div>
  <div class="cell row2-column1">
    <div class="item"></div>
  </div>
  <div class="cell row2-column2">
    <div class="item"></div>
  </div>
</div>
Direction LTR Direction RTL
grid-alignment-example3 grid-alignment-example4

The writing-mode CSS Property applies to the block-flow direction, hence it’s the align-xxx properties the ones affected. In this case, orthogonal writing-modes can be specified in the HTML source code; however, these use cases are not yet fully supported by the current implementation of Grid Layout.

<div class="grid"
      style="align-items: self-start; 
             justify-items: self-start">
  <div class="cell row1-column1">
    <div class="item"></div>
  </div>
  <div class="cell row1-column2">
    <div class="item"></div>
  </div>
  <div class="cell row2-column1">
    <div class="item"></div>
  </div>
  <div class="cell row2-column2">
    <div class="item"></div>
  </div>
</div>
grid-alignment-example3
Vertical LR Vertical RL
grid-alignment-example5 grid-alignment-example6

Technical challenges, accomplished and to be faced

Implementing the Box Alignment specification has been a long task and there is still quite much work ahead for both, WebKit and Blink/Chromium web engines. Perhaps one of the most tedious issue was the definition of a couple of new CSS properties: justify-self and justify-items, which required to touch several Core components, from the CSS parser, the style builder and resolver and finally the rendering.

Another important technical challenge comes from the fact that the Box Alignment properties already present in both web engines were implemented as part of the Flexible Box specification. As it was commented before in this post, the Box Alignment specification aims to generalize the alignment behavior for several layout models, hence these properties were not tied to the Flexible Box implementation anymore; this lead to many technical issue, as I’ll explain later.

The patch implemented for issue 333423005 is a good example of the files to touch and logic to be added in order to implement a new CSS property in Blink/Chromium. There is a similar work to be done in the WebKit web engine; at the time of this writing the similarities are still big, even though some parts changed considerably, like the CSS parsing and style builder logic. As an example, the patch implemented in bug 134419

The following code is quite descriptive of the nature of the CSS Box Alignment properties and how they are applied during the style cascade:

void StyleAdjuster::adjustStyleForAlignment(RenderStyle& style, const RenderStyle& parentStyle)
{
    bool isFlexOrGrid = style.isDisplayFlexibleOrGridBox();
    bool absolutePositioned = style.position() == AbsolutePosition;
 
    // If the inherited value of justify-items includes the legacy keyword, 'auto'
    // computes to the the inherited value.
    // Otherwise, auto computes to:
    //  - 'stretch' for flex containers and grid containers.
    //  - 'start' for everything else.
    if (style.justifyItems() == ItemPositionAuto) {
        if (parentStyle.justifyItemsPositionType() == LegacyPosition) {
            style.setJustifyItems(parentStyle.justifyItems());
            style.setJustifyItemsPositionType(parentStyle.justifyItemsPositionType());
        } else {
            style.setJustifyItems(isFlexOrGrid ? ItemPositionStretch : ItemPositionStart);
        }
    }
 
    // The 'auto' keyword computes to 'stretch' on absolutely-positioned elements,
    // and to the computed value of justify-items on the parent (minus
    // any 'legacy' keywords) on all other boxes (to be resolved during the layout).
    if ((style.justifySelf() == ItemPositionAuto) && absolutePositioned)
        style.setJustifySelf(ItemPositionStretch);
 
    // The 'auto' keyword computes to:
    //  - 'stretch' for flex containers and grid containers,
    //  - 'start' for everything else.
    if (style.alignItems() == ItemPositionAuto)
        style.setAlignItems(isFlexOrGrid ? ItemPositionStretch : ItemPositionStart);
 
    // The 'auto' keyword computes to 'stretch' on absolutely-positioned elements,
    // and to the computed value of align-items on the parent (minus
    // any 'legacy' keywords) on all other boxes (to be resolved during the layout).
    if ((style.alignSelf() == ItemPositionAuto) && absolutePositioned)
        style.setAlignSelf(ItemPositionStretch);
}

The WebKit web engine implements the same logic in the StyleResolver class; the StyleAdjuster class is just a helper class defined in the blink/Chromium engine to assist the StyleReslolver logic during the style cascade in order to make some final adjustmetns.

The issue 297483005 implements the align-self CSS property support in Grid Layout; the follwong code extrated from that patch is a good example of how alingment interacts with the grid tracks.

LayoutUnit RenderGrid::rowPositionForChild(const RenderBox* child) const
{
    bool hasOrthogonalWritingMode = child->isHorizontalWritingMode() != isHorizontalWritingMode();
    ItemPosition alignSelf = resolveAlignment(style(), child->style());
 
    switch (alignSelf) {
    case ItemPositionSelfStart:
        // If orthogonal writing-modes, this computes to 'Start'.
        // FIXME: grid track sizing and positioning does not support orthogonal modes yet.
        if (hasOrthogonalWritingMode)
            return startOfRowForChild(child);
 
        // self-start is based on the child's block axis direction. That's why we need to check against the grid container's block flow.
        if (child->style()->writingMode() != style()->writingMode())
            return endOfRowForChild(child);
 
        return startOfRowForChild(child);
    case ItemPositionSelfEnd:
        // If orthogonal writing-modes, this computes to 'End'.
        // FIXME: grid track sizing and positioning does not support orthogonal modes yet.
        if (hasOrthogonalWritingMode)
            return endOfRowForChild(child);
 
        // self-end is based on the child's block axis direction. That's why we need to check against the grid container's block flow.
        if (child->style()->writingMode() != style()->writingMode())
            return startOfRowForChild(child);
 
        return endOfRowForChild(child);
 
    case ItemPositionLeft:
        // orthogonal modes make property and inline axes to be parallel, but in any case
        // this is always equivalent to 'Start'.
        //
        // self-align's axis is never parallel to the inline axis, except in orthogonal
        // writing-mode, so this is equivalent to 'Start’.
        return startOfRowForChild(child);
 
    case ItemPositionRight:
        // orthogonal modes make property and inline axes to be parallel.
        // FIXME: grid track sizing and positioning does not support orthogonal modes yet.
        if (hasOrthogonalWritingMode)
            return endOfRowForChild(child);
 
        // self-align's axis is never parallel to the inline axis, except in orthogonal
        // writing-mode, so this is equivalent to 'Start'.
        return startOfRowForChild(child);
 
    case ItemPositionCenter:
        return centeredRowPositionForChild(child);
        // Only used in flex layout, for other layout, it's equivalent to 'Start'.
    case ItemPositionFlexStart:
    case ItemPositionStart:
        return startOfRowForChild(child);
        // Only used in flex layout, for other layout, it's equivalent to 'End'.
    case ItemPositionFlexEnd:
    case ItemPositionEnd:
        return endOfRowForChild(child);
    case ItemPositionStretch:
        // FIXME: Implement the Stretch value. For now, we always start align the child.
        return startOfRowForChild(child);
    case ItemPositionBaseline:
    case ItemPositionLastBaseline:
        // FIXME: Implement the ItemPositionBaseline value. For now, we always start align the child.
        return startOfRowForChild(child);
    case ItemPositionAuto:
        break;
    }
 
    ASSERT_NOT_REACHED();
    return 0;
}

The resolveAlignment function call deserves an special mention, since it will lead to the open issues I’m still working on. The Box Alignment specification states that the auto values must be resolved to either stretch or start depending on the kind of element. This is theoretically performed during the style cascade, so it wouldn’t be necessary to resolve it at the rendering stage. The code is pretty simple :

static ItemPosition resolveAlignment(const RenderStyle* parentStyle, const RenderStyle* childStyle)
{
    ItemPosition align = childStyle->alignSelf();
    // The auto keyword computes to the parent's align-items computed value, or to "stretch", if not set or "auto".
    if (align == ItemPositionAuto)
        align = (parentStyle->alignItems() == ItemPositionAuto) ? ItemPositionStretch : parentStyle->alignItems();
    return align;
}

The RenderFlexibleBox implementation has to define a similar logic and what is more important, the default value of all the Box Alignment properties have been changed to auto, instead of stretch as it’s stated in the Flexbible Box specification.

To make things even more complicated, many HTML elements are being rendered by RenderFlexibleBox objects as an implementation decision, without the proper display value set to indicate such assumption. This causes many issues and layout tests failures, since the resolved value for auto depends on the kind of element, which is defined by its display property value. Additionally, there are also problems with the anonymous render objects added to the tree on certain implementations.

Both WebKit and Blink/Chromium are affected by these issues; Mathml is a good example for the WebKit engine, since most if its render objects are implemented using a RenderFlexibleBox; also, it assigns and manipulates the align-{self, items} properties during the layout. The RenderFullScreen object is a source of problems for the Blink/Chromium web engine on this regard; it uses a RenderFleixibleBox because of its stretch default behavior, which is not the case anymore according to the Box Alignment specification.

I’m still working on theses issues in both web engines, so this issue is trying to face part of the problems on Blink/Chromium. There are a similar bug in the WebKit engine with similar challenges.

Another pending issue present in both web engines is the lack of support for different writing-modes. Eventhouth the Grid Layout logic is prepared to support them, it’s still buggy and for certain combinations it does not produce the expected outcome.

I’d like to finish this post pointing out that anybody can follow the progress of the Box Alignment spec implementation for Grid Layout you can track these bugs on either of the web engine you are more interested on:

  • Blink/Chromium
    • bug 249451: [CSS Grid Layout] Implement row-axis Alignment
    • bug 376823: [CSS Grid Layout] Implement column-axis Alignment
  • WebKit
    • bug 133224 – [meta] [CSS Grid Layout] Implement column-axis Alignment
    • bug 133222 – [meta] [CSS Grid Layout] Implement row-axis Alignment

This work wouldn’t be possible without the support of Bloomberg and Igalia, who are comitted to provide a better web platform for developers.

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