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Re: Selector for tags with a certain child.

From: Simetrical <simetrical@gmail.com>
Date: Wed, 8 Oct 2008 18:01:12 -0400
Message-ID: <7c2a12e20810081501i73be96c3l8de1f91b9eb5452b@mail.gmail.com>
To: "Keiji Ikari" <kei@teamikaria.com>
Cc: "W3C Style List" <www-style@w3.org>

On Wed, Oct 8, 2008 at 3:52 PM, Keiji Ikari <kei@teamikaria.com> wrote:
> What I would like to be able to do, though, is set a CSS rule which,
> by some selector, would specify "all <a> tags containing an <img>
> tag". It would have the effect of using the selector "a img" but then
> applying the style to the parent of the <img> tag, not the <img> tag
> itself. This would prevent me from having to modify the classes of
> every image that was inside a link.
>
> On a perhaps somewhat related note, I'd also like to suggest a
> ":child" generated content selector, similar to ":before" and
> ":after", except this time behaving as if a <div>...</div> (or
> <span>...</span> for inline elements) were inserted around all the
> content of the tag following the rule. In a lot of situations I need
> to always nest one tag inside another and apply the same style to this
> nested tag, which leads to HTML being more complicated than it needs
> to be.

This has been discussed extensively: for a recent occasion, see
<http://lists.w3.org/Archives/Public/www-style/2008Jul/0442.html> and
the dozens of replies to that.  The conclusion is that such a selector
would be much more expensive to implement than a parent selector,
because it would require reflows during document layout.

In your case, the browser would be forced to begin rendering the <a>
element, then hit the image and have to erase what it already did and
start over.  Or else to not even bother rendering the <a> element
until it verified that it had no <img> children, which could mean that
display is frozen for a relatively long time, until it finds the </a>
(however long that is).

These might seem worth it, but consider a rule matching "every body
element containing a div with class 'abc'".  Consider *multiple* such
rules.  Such a rule would require the entire document to be reflowed,
maybe multiple times, as part of incremental layout.  This would be
noticeably slow and confusing to users, and quite unpleasant.

Therefore, CSS requires the document author to take responsibility for
using classes, so that the rendering agent can know in advance what it
has to render.  This allows documents to render faster and more
smoothly.  UAs are evidently not willing to take the performance hit
that arbitrary parent selectors could cause in a poorly-optimized
document.
Received on Wednesday, 8 October 2008 22:01:48 GMT

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