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POWhich CSS selectors or rules can significantly affect front-end layout / rendering performance in the real world?
 

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  1. POWhich CSS selectors or rules can significantly affect front-end layout / rendering performance in the real world?
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    <p><strong>Is it worth worrying about CSS rendering performance?</strong> Or should we just not worry about efficiency at all with CSS and just focus on writing elegant or maintainable CSS instead?</p> <p>This question is intended to be a useful resource for front-end developers on which parts of CSS can actually have a significant impact on device performance, and which devices / browsers or engines may be affected. This is <em>not</em> a question about how to write elegant or maintainable CSS, it's purely about performance (although hopefully what's written here can inform more general articles on best-practice).</p> <h1>Existing evidence</h1> <p><a href="https://developers.google.com/speed/docs/best-practices/rendering" rel="noreferrer">Google</a> and <a href="https://developer.mozilla.org/en-US/docs/CSS/Writing_Efficient_CSS?redirectlocale=en-US&amp;redirectslug=Writing_Efficient_CSS" rel="noreferrer">Mozilla</a> have written guidelines on writing efficient CSS and <a href="http://csslint.net/about.html" rel="noreferrer">CSSLint's set of rules</a> includes:</p> <blockquote> <p>Avoid selectors that look like regular expressions .. don't use the complex equality operators to avoid performance penalties</p> </blockquote> <p>but none of them provide any evidence (that I could find) of the impact these have.</p> <p>A <a href="http://css-tricks.com/efficiently-rendering-css/" rel="noreferrer">css-tricks.com article on efficient CSS</a> argues (after outlining a load of efficiency best practices) that we should <code>not .. sacrifice semantics or maintainability for efficient CSS</code> these days.</p> <p>A <a href="http://perfectionkills.com/profiling-css-for-fun-and-profit-optimization-notes/#summary" rel="noreferrer">perfection kills blog post</a> suggested that <code>border-radius</code> and <code>box-shadow</code> rendered orders of magnitude slower than simpler CSS rules. This was hugely significant in Opera's engine, but insignificant in Webkit. Further, a <a href="http://coding.smashingmagazine.com/2011/04/21/css3-vs-css-a-speed-benchmark/" rel="noreferrer">smashing magazine CSS benchmark</a> found that rendering time for CSS3 display rules was insignificant and significantly faster than rendering the equivalent effect using images.</p> <p>Know your mobile <a href="http://www.knowyourmobile.com/features/1464976/google_chrome_for_ios_vs_safari_an_indepth_speed_and_feature_test.html" rel="noreferrer">tested various mobile browsers</a> and found that they all rendered CSS3 equally insignificantly fast (in 12ms) but it looks like they did the tests on a PC, so we can't infer anything about how hand-held devices perform with CSS3 in general.</p> <p>There <a href="https://developers.google.com/speed/docs/best-practices/rendering" rel="noreferrer">are</a> <a href="https://developers.google.com/speed/docs/best-practices/rendering" rel="noreferrer">many</a> <a href="https://developer.mozilla.org/en-US/docs/CSS/Writing_Efficient_CSS?redirectlocale=en-US&amp;redirectslug=Writing_Efficient_CSS" rel="noreferrer">articles</a> on the internet on how to write efficient CSS. However, I have yet to find any comprehensive evidence that badly considered CSS actually has a <em>significant</em> impact on the rendering time or snappiness of a site.</p> <h1>Background</h1> <p><a href="http://brightgreenscotland.org/wp-content/uploads/2010/09/stand-back-Im-going-to-try-science.png" rel="noreferrer">stand back, I&#39;m going to try science http://brightgreenscotland.org/wp-content/uploads/2010/09/stand-back-Im-going-to-try-science.png</a></p> <p>I offered bounty for this question to try to use the community power of SO to create a useful well-researched resource.</p>
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    1. USRobin Winslow
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    1. COOne thing I can surely tell you: use IDs when IDs should be used, and classes when classes should be used. Performance difference is negligible, semantics isn't. IDs for elements which — by definition — appear only once; classes for those which can repeat throughout the page. Just consider the extreme case when you use a class for a `fixed` CSS position.
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      1. POWhich CSS selectors or rules can significantly affect front-end layout / rendering performance in the real world?
      1. USMichał Górny
    2. CO@MichałGórny IDs should be used in *markup* where they are appropriate, but many people believe (myself included) that IDs should never be used in CSS selectors. Read this article for a (hopefully unbiased) elaboration: http://screwlewse.com/2010/07/dont-use-id-selectors-in-css/
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      1. USRobin Winslow
    3. COWell, that article agrees with me on when IDs can and should be used. And my extreme example of `position: fixed` is an example when CSS simply shouldn't be reused. Not that I'm advocating on doing something like that.
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      1. USMichał Górny
 

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