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Re: {css-3 selectors} The :parent pseudo-class 2

From: Tab Atkins Jr. <jackalmage@gmail.com>
Date: Tue, 24 Mar 2009 07:25:46 -0500
Message-ID: <dd0fbad0903240525j4314ef25u1d59aea5928ea9a@mail.gmail.com>
To: Gabriele Romanato <gabriele.romanato@gmail.com>
Cc: www-style@w3.org
2009/3/23 Gabriele Romanato <gabriele.romanato@gmail.com>:
> For more specificity, we could write this pseudo-class as:
>
> E:parent(E1)
>
> Where E1 is the parent element:
>
> a:parent(h3) {...}

CSS has had many proposals for a pseudoclass that goes 'in the
opposite direction', and selects an element based on its
children/descendants (or previous siblings).

Unfortunately, in the general case it's a desperately expensive
operation.  Now that I've learned much more about selector engines and
the way they operate (and *why* they operate that way), that sort of
operation will pretty much *always* be extremely expensive.  It can be
reasonably implemented on a complete DOM tree, which is why javascript
libraries like jQuery can do it, but it can't be done quickly on a DOM
that is being built element-by-element as the HTML parser reads a file
over the wire.  Unfortunately this latter case is *by far* the most
common cause of selector matching.  In general, you need to be able to
quickly grab all the selectors that can even *possibly* apply to a
newly added or mutated element, and then walk up the tree to verify
which ones actually do.  Walking *down* the tree is enormously more
expensive, as should be obvious (an element has a single parent, but
possibly many children).

That said, it appears, based on feedback from implementers, that
implementing merely the parent selector, and perhaps the previous
sibling selector, wouldn't be horrendous.  It would still be slower
than the existing simple selectors (even something that *seems*
symmetric, like matching previous-sibling vs next-sibling, really
isn't when you're consuming a document node by node).

However, your syntax simply doesn't work.  As you have it, you are
targeting an <a> element, *not* the parent, and certainly not an <h3>
element.  The closest you could get is that you're targetting an <a>
which has an <h3> as a parent, which is more simply written as "h3 >
a".  Pseudoclasses are just what they sound like - imaginary classes
applied to elements based on some property they hold.

You have to target the parent first, then select it based on its
children.  In the simple case, something like "h3:child(a)".  In a
more general form, "h3:matches( > a )".  The latter has the advantage
of being extensible when we find a good way to match more generally
(or just get performant enough that we stop caring), but the former is
much simpler for authors and doesn't have the 'unbalanced selector'
that appears to cause problems for some.

~TJ
Received on Tuesday, 24 March 2009 12:26:23 GMT

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