Improving CSS Custom Properties performance

Chrome 84 reached the stable channel a few weeks ago, and there are already several great posts describing the many important additions, interesting new features, security fixes and improvements in privacy policies (([1], [2], [3], [4]) it contains. However, there is a change that I worked on in this release which might have passed unnoticed by most, but I think is very valuable: A change regarding CSS Custom Properties (variables) performance.

The design of CSS, in general, takes great care in considering how features are designed with respect to making it possible for them to perform well. However, implementations may not perform as well as they could, and it takes a considerable amount of time to understand how authors use the features and which cases are more relevant for them.

CSS Custom Properties are an interesting example to look at here: They are a wonderful feature that provides a lot of advantages for web authors. For a whole lot of cases, all of the implementations of CSS Custom Properties perform well enough that most people won\’t notice. However, we at Igalia have been analyzing several use cases and looking at some reports around their performance in different implementations.

Let\’s consider a fairly straightforward example in which an author sets a single property in a toggleable class in the body, and then uses that property several times deeper in the tree to change the foreground color of some text.




Only about 20% of those actually use this property, 5 elements deep into the tree, and only to change the foreground color.

To evaluate Chromium\’s performance in a case like this we can define a new perf tests, using the perf tools the Chromium project has available for browser engineers. In this case, we want a huge tree so that we can evaluate better the impact of the different optimizations.



    

These are the results obtained runing the test in Chrome 83:

avg median stdev min max
163.74 ms 163.79 ms 3.69 ms 158.59 ms 163.74 ms

I admit that it\’s difficult to evaluate the results, especially considering the number of nodes of such a huge DOM tree. Lets compare the results of the same test on Firefox, using different number of nodes.

Nodes 50K 20K 10K 5K 1K 500
Chrome 83 163.74 ms 55.05 ms 25.12 ms 14.18 ms 2.74 ms 1.50 ms
FF 78 28.35 ms 12.05 ms 6.10 ms 3.50 ms 1.15 ms 0.55 ms
1/6 1/5 1/4 1/4 1/2 1/3

As I commented before, the data are more accurate when the DOM tree has a lot of nodes; in any case, the difference is quite clear and shows there is plenty room for improvement. WebKit based browsers have results more similar to Chromium as well.

Performance tests like the one above can be added to browsers for tracking improvements and regressions over time, so we\’ve added (r763335) that to Chromium\’s tree: We\’d like to see it get faster over time, and definitely cannot afford regressions (see Chrome Performance Dashboard and the ChangeStyleCustomPropertyDeclaration test for details) .

So… What can we do?

In Chrome 83 and lower, whenever the custom property declaration changed, the new declaration would be inherited by the whole tree. This inheritance implied executing the whole CSS cascade and recalculating the styles of all the nodes in the entire tree, since with this approach, all nodes may be affected.

Chrome had already implemented an optimization on the CSS cascade implementation for regular CSS properties that don’t depend on any other to resolve their value. These subset of CSS properties are defined as Independent Properties in the Chromium codebase. The optimization mentioned before affects how the inheritance mechanism is implemented for these Independent properties. Whenever one of these properties changes, instead of recalculating the styles of the inherited properties, children can just copy the whole parent’s computed style. Blink\’s style engine has a component known as Matched Properties Cache responsible of deciding when is possible to avoid the style resolution of an element and instead, performing an efficient copy of the matched computed style. I\’ll get back to this concept in the last part of this post.

In the case of CSS Custom Properties, we could apply a similar approach as a good step. We can consider that the nodes with computed styles that don\’t have references to custom properties declarations shouldn’t be affected by the new declaration, and we can implement the inheritance directly by copying the parent’s computed style. The patch with the optimization I\’ve implemented in r765278 initially landed in Chrome 84.0.4137.0

Let\’s look at the result of this one action in the Chrome Performance Dashboard:

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That\’s a really good improvement!

However, it\’s also just a first step. It’s clear that Chrome still has a wide margin for improvement in this case, as well any WebKit based browser – Firefox is still, impressively, markedly faster as it\’s been described in the bug report filed to track this issue. The following table shows the result of the different browsers together; even disabling the muti-thread capabilities of Firefox\’s Stylo engine (STYLO_THREAD=1), FF is much faster than Chrome with the optimization applied.

Chrome 83 Chrome 84 FF 78 FF 78 th=1
avg
median
stdev
min
max
163.74 ms
163.79 ms
3.69 ms
158.59 ms
163.74 ms
117.37 ms
117.52 ms
1.98 ms
113.66 ms
120.87 ms
28.35 ms
28.50 ms
0.93 ms
26.00 ms
30.00 ms
38.25 ms
38.50 ms
1.86 ms
35.00 ms
41.00 ms

Before continue, I want get back to the Matched Properties Cache (MPC) concept, since it has an important role on these style optimizations. This cache is not a new concept in the Chrome\’s engine; as a matter of fact, it\’s also used in WebKit, since it was implemented long ago, before the fork that created the new blink engine. However, Google has been working a lot on this area in the last years and some of the most recent changes in the MPC have had an important impact on style resolution performance. As a result of this work, elements with independent and non-independent properties using CSS Variables might produce cache hits in the MPC. The results of the Performance Dashboard show a considerable improvement in the mentioned ChangeStyleCustomPropertyDeclaration test (avg: 108.06 ms)

Additionally, there are several other cases where the use of CSS Variables has a considerable impact on performance, compared with using regular CSS properties. Obviously, resolving CSS Variables has a cost, so it\’s clear that we could apply additional optimizations that reduce the impact of the variable resolution, especially for handling specific style changes that might not affect to a substantial portion of the DOM tree. I\’ve been experimenting with the MPC to explore the idea an independent CSS Custom Properties cache; nodes with variables referencing the same custom property will produce cache hits in the MPC, even though other properties don\’t match. The preliminary approach I\’ve been implementing consists on a new matching function, specific for custom properties, and a mechanism to transfer/copy the property\’s data to avoid resolving the variable again, since the property\’s declaration hasn\’t change. We would need to apply the css cascade again, but at least we could save the cost of the variable resolution.

Of course, at the end of the day, improving performance has costs and challenges – and it\’s hard to keep performance even once you get it. Bit if we really want performant CSS Custom Properties, this means that we have to decide to prioritize this work. Currently there is reluctance to explore the concept of a new Custom Properties specific cache – the challenge is big and the risks are not non-existent; cache invalidation can get complicated. But, the point is that we have to understand that we aren\’t all going to agree what is important enough to warrant attention, or how much investment, or when. Web authors must convince vendors that these use cases are worth being optimized and that the cost and risks of such a complex challenges should be assumed by them.

This work has been sponsored by Bloomberg, which I consider one of the most important contributors of the Web Platform. After several years, the vision of this company and its responsibility as consumer of the platform has lead to many and important contributions that we all enjoy now. Although CSS Grid Layout might be the most remarkable one, there are may other not that big, like this work on CSS Custom Properties, or several other new features of the CSS Text specification. This is a perfect example of an company that tries to change priorities and adapt the web platform to its needs and the use cases they consider more aligned with their business strategy.

I understand that not every user of the web platform can do this kind of investment. This is why I believe that initiatives like Open Priorization could help to move the web platform in a positive direction. By providing a way for us to move past a lot of these conversation and focus on the needs that some web authors and users of the platform consider more important, or higher priority. Improving performance for CSS Custom Properties isn\’t currently one of the projects we\’ve listed, but perhaps it would be an interesting one we might try in the future if we are successful with these. If you haven\’t already, have a look and see if there is something there that is interesting to you or your company – pledges of any size are good – ten thousand $1 donations are every bit as good as ten $1000 donations. Together, we can make a difference, and we all benefit.

Also, we would love to hear about your ideas. Is improving CSS Custom Properties performance important to you? What else is? Share your comments with us on Twitter, either me (@lajava77) or our developer advocate Brian Kardell (@briankardell), or email me at jfernandez@igalia.com. I\’d be glad to answer any question about the Open Priorization experiment.

A new terminal-style line breaking with CSS Text

The CSS Text 3 specification defines a module for text manipulation and covers, among a few other features, the line breaking behavior of the browser, including white space handling. I\’ve been working lately on some new features and bug fixing for this specification and I\’d like to introduce in this posts the last one we made available for the Web Platform users. This is yet another contribution that came out the collaboration between Igalia and Bloomberg, which has been held for several years now and has produced many important new features for the Web, like CSS Grid Layout.

The feature

I guess everybody knows the white-space CSS property, which allows web authors to control two main aspects of the rendering of a text line: collapsing and wrapping. A new value break-spaces has been added to the ones available for this property, which allows web authors to emulate a terminal-like line breaking behavior. This new value operates basically like pre-wrap, but with two key differences:

  • any sequence of preserved white space characters takes up space, even at the end of the line.
  • a preserved white space sequence can be wrapped at any character, moving the rest of the sequence, intact, to the line bellow.

What does this new behavior actually mean ? I\’ll try to explain it with a few examples. Lets start with a simple but quite illustrative demo which tries to emulate a meteorology monitoring system which shows relevant changes over time, where the gaps between subsequent changes must be preserved:




    

Another interesting use case for this feature could be a logging system which should preserve the text formatting of the logged information, considering different window sizes. The following demo tries to describe this such scenario:



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Use cases

In the demo shown before there are several cases that I think it\’s worth to analyze in detail.

A breaking opportunity exists after any white space character

The main purpose of this feature is to preserve the white space sequences length even when it has to be wrapped into multiple lines. The following example tries to describe this basic use case:



XX XX

The example above shows how the white space sequence with a length of 15 characters is preserved and wrapped along 3 different lines.

Single leading white space

Before the addition of the break-spaces value this scenario was only possible at the beginning of the line. In any other case, the trailing white spaces were either collapsed or hang, hence the next line couldn\’t start with a sequence of white spaces. Lets consider the following example:



XX XX

Like when using pre-wrap, the single leading space is preserved. Since break-spaces allows breaking opportunities after any white space character, we break after the first leading white space (\” |XX XX\”). The second line can be broken after the first preserved white space, creating another leading white space in the next line (\” |XX | XX\”).

However, lets consider now a case without such first single leading white space.



XXX XX

Again, it s not allowed to break before the first space, but in this case there isn\’t any previous breaking opportunity, so the first space after the word XX should overflow (\”XXX | XX\”); the next white space character will be moved down to the next line as preserved leading space.

Breaking before the first white space

I mentioned before that the spec states clearly that the break-space feature allows breaking opportunities only after white space characters. However, it\’d be possible to break the line just before the first white space character after a word if the feature is used in combination with other line breaking CSS properties, like word-break or overflow-wrap (and other properties too).



XXXX X

The two white spaces between the words are preserved due to the break-spaces feature, but the first space after the XXXX word would overflow. Hence, the overflow-wrap: break-word feature is applied to prevent the line to overflow and introduce an additional breaking opportunity just before the first space after the word. This behavior causes that the trailing spaces are moved down as a leading white space sequence in the next line.

We would get the same rendering if word-break: break-all is used instead overflow-wrap (or even in combination), but this is actualy an incorrect behavior, which has the corresponding bug reports in WebKit (197277) and Blink (952254) according to the discussion in the CSS WG (see issue #3701).

Consider previous breaking opportunities

In the previous example I described a combination of line breaking features that would allow breaking before the first space after a word. However, this should be avoided if there are previous breaking opportunities. The following example is one of the possible scenarios where this may happen:



XX X X

In this case, we could break after the second word (\”XX X| X\”), since overflow-wrap: break-word would allow us to do that in order to avoid the line to overflow due to the following white space. However, white-space: break-spaces only allows breaking opportunities after a space character, hence, we shouldn\’t break before if there are valid previous opportunities, like in this case in the space after the first word (\”XX |X X\”).

This preference for previous breaking opportunities before breaking the word, honoring the overflow-wrap property, is also part of the behavior defined for the white-space: pre-wrap feature; although in that case, there is no need to deal with the issue of breaking before the first space after a word since trailing space will just hang. The following example uses just the pre-wrap to show how previous opportunities are selected to avoid overflow or breaking a word (unless explicitly requested by word-break property).



XX
overflow-wrap:
break-word
word-break:
break-all

In this case, break-all enables breaking opportunities that are not available otherwise (we can break a word at any letter), which can be used to prevent the line to overflow; hence, the overflow-wrap property doesn\’t take any effect. The existence of previous opportunities is not considered now, since break-all mandates to produce the longer line as possible.

This new white-space: break-spaces feature implies a different behavior when used in combination with break-all. Even though the preference of previous opportunities should be ignored if we use the word-break: break-all, this may not be the case for the breaking before the first space after a word scenario. Lets consider the same example but using now the word-break: break-all feature:



XX X X

The example above shows that using word-break: break-all doesn\’t produce any effect. It\’s debatable whether the use of break-all should force the selection of the breaking opportunity that produces the longest line, like it happened in the pre-wrap case described before. However, the spec states clearly that break-spaces should only allow breaking opportunities after white space characters. Hence, I considered that breaking before the first space should only happen if there is no other choice.

As a matter of fact, specifying break-all we shouldn\’t considering only previous white spaces, to avoid breaking before the first white space after a word; the break-all feature creates additional breaking opportunities, indeed, since it allows to break the word at any character. Since break-all is intended to produce the longest line as possible, this new breaking opportunity should be chosen over any previous white space. See the following test case to get a clearer idea of this scenario:



X XX X

Bear in mind that the expected rendering in the above example may not be obtained if your browser\’s version is still affected by the bugs 197277(Safari/WebKit) and 952254(Chrome/Blink). In this case, the word is broken despite the opportunity in the previous white space, and also avoiding breaking after the \’XX\’ word, just before the white space.

There is an exception to the rule of avoiding breaking before the first white space after a word if there are previous opportunities, and it\’s precisely the behavior the line-break: anywhere feature would provide. As I said, all these assumptions were not, in my opinion, clearly defined in the current spec, so that\’s why I filed an issue for the CSS WG so that we can clarify when it\’s allowed to break before the first space.

Current status and support

The intent-to-ship request for Chrome has been approved recently, so I\’m confident the feature will be enabled by default in Chrome 76. However, it\’s possible to try the feature in older versions by enabling the Experimental Web Platform Features flag. More details in the corresponding Chrome Status entry. I want to highlight that I also implemented the feature for LayoutNG, the new layout engine that Chrome will eventually ship; this achievement is very important to ensure the stability of the feature in future versions of Chrome.

In the case of Safari, the patch with the implementation of the feature landed in the WebKit\’s trunk in r244036, but since Apple doesn\’t announce publicly when a new release of Safari will happen or which features it\’ll ship, it\’s hard to guess when the break-spaces feature will be available for the web authors using such browser. Meanwhile, It\’s possible to try the feature in the Safari Technology Preview 80.

Finally, while I haven\’t see any signal of active development in Firefox, some of the Mozilla developers working on this area of the Gecko engine have shown public support for the feature.

The following table summarizes the support of the break-spaces feature in the 3 main browsers:

Chrome Safari Firefox
Experimental M73 STP 80 Public support
Ship M76 Unknown Unknown

Web Platform Tests

At Igalia we believe that the Web Platform Tests project is a key piece to ensure the compatibility and interoperability of any development on the Web Platform. That\’s why a substantial part of my work to implement this relatively small feature was the definition of enough tests to cover the new functionality and basic use cases of the feature.

white-space overflow-wrap word-break
pre-wrap-008
pre-wrap-015
pre-wrap-016
break-spaces-003
break-spaces-004
break-spaces-005
break-spaces-006
break-spaces-007
break-spaces-008
break-spaces-009
break-word-004
break-word-005
break-word-006
break-word-007
break-word-008
break-all-010
break-all-011
break-all-012
break-all-013
break-all-014
break-all-015

Implementation in several web engines

During the implementation of a browser feature, even a small one like this, it\’s quite usual to find out bugs and interoperability issues. Even though this may slow down the implementation of the feature, it\’s also a source of additional Web Platform tests and it may contribute to the robustness of the feature itself and the related CSS properties and values. That\’s why I decided to implement the feature in parallel for WebKit (Safari) and Blink (Chrome) engines, which I think it helped to ensure interoperability and code maturity. This approach also helped to get a deeper understanding of the line breaking logic and its design and implementation in different web engines.

I think it\’s worth mentioning some of these code architectural differences, to get a better understanding of the work and challenges this feature required until it reached web author\’s browser.

Chrome/Blink engine

Lets start with Chrome/Blink, which was especially challenging due to the fact that Blink is implementing a new layout engine (LayoutNG). The implementation for the legacy layout engine was the first step, since it ensures the feature will arrive earlier, even behind an experimental runtime flag.

The legacy layout relies on the BreakingContext class to implement the line breaking logic for the inline layout operations. It has the main characteristic of handling the white space breaking opportunities by its own, instead of using the TextBreakIterator (based on ICU libraries), as it does for determining breaking opportunities between letters and/or symbols. This design implies too much complexity to do even small changes like this, especially because is very sensible in terms of performance impact. In the following diagram I try to show a simplified view of the classes involved and the interactions implemented by this line breaking logic.

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The LayoutNG line breaking logic is based on a new concept of fragments, mainly handled by the NGLineBreaker class. This new design simplifies the line breaking logic considerably and it\’s highly optimized and adapted to get the most of the TextBreakIterator classes and the ICU features. I tried to show a simplified view of this new design with the following diagram:

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In order to describe the work done to implement the feature for this web engine, I\’ll list the main bugs and patches landed during this time: CR#956465, CR#952254, CR#944063,CR#900727, CR#767634, CR#922437

Safari/WebKit engine

Although as time passes this is less probable, WebKit and Blink still share some of the layout logic from the ages prior to the fork. Although Blink engineers have applied important changes to the inline layout logic, both code refactoring and optimizations, there are common design patterns that made relatively easy porting to WebKit the patches that implemented the feature for the Blink\’s legacy layout. In WebKit, the line breaking logic is also implemented by the BreakingContext class and it has a similar architecture, as it\’s described, in a quite simplified way, in the class diagram above (it uses different class names for the render/layout objects, though) .

However, Safari supports for the mac and iOS platforms a different code path for the line breaking logic, implemented in the SimpleLineLayout class. This class provides a different design for the line breaking logic, and, similar to what Blink implements in LayoutNG, is based on a concept of text fragments. It also relies as much as possible into the TextBreakIterator, instead of implementing complex rules to handle white spaces and breaking opportunities. The following diagrams show this alternate design to implement the line breaking process.

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This SimpleLineLayout code path in not supported by other WebKit ports (like WebKitGtk+ or WPE) and it\’s not available either when using some CSS Text features or specific fonts. There are other limitations to use this SimpleLineLayout codepath, which may lead to render the text using the BreakingContext class.

Again, this is the list of bugs that were solved to implement the feature for the WebKit engine: WK#197277, WK#196169, WK#196353, WK#195361, WK#177327, WK#197278

Conclusion

I hope that at this point these 2 facts are clear now:

  • The white-space: break-spaces feature is a very simple but powerful feature that provides a new line breaking behavior, based on unix-terminal systems.
  • Although it\’s a simple feature, on the paper (spec), it implies a considerable amount of work so that it reaches the browser and it\’s available for web authors.

In this post I tried to explain in a simple way the main purpose of this new feature and also some interesting corner cases and combinations with other Line Breaking features. The demos I used shown 2 different use cases of this feature, but there are may more. I\’m sure the creativity of web authors will push the feature to the limits; by then, I\’ll be happy to answer doubts, about the spec or the implementation for the web engines, and of course fix the bugs that may appear once the feature is more used.

\"Igalia
\"Bloomberg

Igalia and Bloomberg working together to build a better web

Finally, I want to thank Bloomberg for supporting the work to implement this feature. It\’s another example of how non-browser vendors can influence the Web Platform and contribute with actual features that will be eventually available for web authors. This is the kind of vision that we need if we want to keep a healthy, open and independent Web Platform.