W3C home > Mailing lists > Public > www-style@w3.org > April 2012

Re: [css-selectors] :contains()

From: Tab Atkins Jr. <jackalmage@gmail.com>
Date: Thu, 26 Apr 2012 10:29:47 -0700
Message-ID: <CAAWBYDCmDm57DHH_d0XdpJBniX=GBQGW42+vSnwihxHN3rE-eA@mail.gmail.com>
To: Sebastian Zartner <sebastianzartner@gmx.de>
Cc: www-style@w3.org
On Thu, Apr 26, 2012 at 12:33 AM, Sebastian Zartner
<sebastianzartner@gmx.de> wrote:
>> For general CSS, though, you have to match *dynamically* - every time
>> the document is changed, you must quickly find which selectors stop
>> matching, and which new selectors start matching.  This is *not* a
>> trivial task, and it has to be done *quickly*, so that things like
>> :hover feel responsive.  Some features make this much harder, like
>> :contains() and especially the subject selector.  It might be possible
>> to do dynamic matching quickly, but it's a non-trivial task.
> Sure, but if no one tries, we won't get any further. Also the use cases for :hover and :contains() are quite different. While reacting to mouse moves must be instant reacting to text changes does not forcibly.

You're assuming here that the two pseudo-classes are used separately.
They can show up in the same selector, you know.  ^_^

>> And what's worse, the mere *presence* of one of these slow rules can
>> potentially slow down matching for the entire page, even if it never
>> actually matches anything.
> Yes, but that counts for all rules. Though of course by having slow rules you'll realize the decrease in page speed faster.

Not really.  Performance is often a black art; selector performance
doubly so.  The browser's Inspectors are finally starting to peel away
the veil here, but it's still going to be something learned mostly
through rumors and out-of-date tests, and most people won't care at
all.  You might be able to tell "hey, my page is slow", but figuring
out *why* is much harder.

Plus, a particular slow rule might not make a noticeable difference on
the brand-new Macbook the author is using, but create a really choppy
experience on an older desktop or a phone.  It's like the difference
between using "box-shadow: 2px 2px black;" and "box-shadow: 2px 2px
1px black;".  It looks so small, but that little blur has a *huge*
effect on page performance.  It's one of the primary footguns on the
web today, unfortunately, where pages run slow for no good reason just
because an author put a little bit of blur in their shadows, and
*their* computer was fast enough that they didn't notice the problem.

Received on Thursday, 26 April 2012 17:30:43 UTC

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