The final test (https://benfrain.com/selector-test/3-01.html) I ran was to hit the page with a bunch of expensive properties and values. Consider this rule:
.link {
background-color: red;
border-radius: 5px;
padding: 3px;
box-shadow: 0 5px 5px #000;
-webkit-transform: rotate(10deg);
-moz-transform: rotate(10deg);
-ms-transform: rotate(10deg);
transform: rotate(10deg);
display: block;
}
With that rule applied, here are the results:
|
Test |
Chrome 34 |
Firefox 29 |
Opera 19 |
IE 19 |
Android 4 |
|
Expensive Styles |
65.2 |
151.4 |
65.2 |
259.2 |
1923 |
Here all browsers are at least up with their slowest selector speed (IE was 1.5X slower than its slowest selector test (10) and the Android device was 1.3X slower than the slowest selector test (test 6)) but that's not even the full picture. Try and scroll that page! Repaint on those kind of styles can bring a browser to its knees (or whatever the equivalent of knees is for a browser).
The properties we stick inside the braces are what really affects performance. It stands to reason that scrolling a page that requires endless expensive re-paints and layout changes is going to put a strain on the device. Nice HiDPI screen? It will be even worse as the CPU/GPU strains to get everything re-painted to screen in under 16 ms.
With the expensive styles test, on the 15" Retina MacBook Pro I tested on, the paint time shown in continuous paint mode in Chrome never dropped below 280 ms (and remember, we are aiming for sub–16 ms). To put that in perspective for you, the first selector test page, never went above 2.5 ms. That wasn't a typo. Those properties created a 112X increase in paint time. Holy expensive properties Batman! Indeed Robin. Indeed.
An expensive property/value pairing is one we can be pretty confident will make the browser struggle with when it has to repaint the screen (e.g. on scroll).
How can we know what will be an expensive style? Thankfully, we can apply common sense to this and get a pretty good idea what is going to tax the browser. Anything that requires a browser to manipulate/calculate before painting to the page will be more costly. For example, box-shadows, border-radius, transparency (as the browser has to calculate what is shown below), transforms and performance killers like CSS filters - if performance is your priority, anything like that is your worst enemy.
Note
Juriy kangax Zaytsev did a fantastic blog post also covering CSS performance (http://perfectionkills.com/profiling-css-for-fun-and-profit-optimization-notes/) back in 2012. He was using the various developer tools to measure performance. He did a particularly good job of showing the difference that various properties had on performance. If this kind of thing interests you then that post is well worth your time.