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Re: proposal for a new css combinator

From: Tab Atkins Jr. <jackalmage@gmail.com>
Date: Thu, 14 Jan 2010 10:40:05 -0600
Message-ID: <dd0fbad1001140840p6a57abcnb12b5419733270cb@mail.gmail.com>
To: Alan Gresley <alan@css-class.com>
Cc: √ėyvind Stenhaug <oyvinds@opera.com>, www-style list <www-style@w3.org>
On Thu, Jan 14, 2010 at 10:31 AM, Alan Gresley <alan@css-class.com> wrote:
> Structural pseudo-class are different. :first-child and :last-child selects
> the first or last element of a given type. With Niels Matthijs proposal this
> was for <header> but this could a <p> or a <div>. Unlike the structural
> pseudo-classes of :first-child and :last-child which goes across a
> particular generation of the DOM, :first-descendant and :last-descendant
> would go up or down the tree.

Your examples are correct, but your analogy tying :first-child and
:first-descendant together are not.

"p :first-child" does *not* select the first child of a p element.
Well, it does, but not *just* that.  Given this code:

<p>
  <span>
    <i>foo</i>
  </span>
</p>

Both the span *and* the i are selected, since they are both
first-children who are descendants of p.  The :first-child pseudoclass
cares only about the DOM, *not* about the selector, and thus does not
tie together the matched element and the p in any way - the only link
between them is through the descendant combinator.

On the other hand, a first-descendant matches *does* care about the
selector.  "p i:first-descendant" will only match an i if there is no
other i between it and the p ancestor.  To function, it must know
where to start and stop searching for additional i's, and it can only
know that by having knowledge about the selector itself.

Thus first-descendant is appropriate as a combinator, but not as a
pseudoclass, given the way that the lines between them are currently
drawn.  That's not to say that it must always be this way - it's
possible that having a pseudoclass which has knowledge of the selector
it's used in would be useful enough to add - but for now I think it
best to stick to the categories as they have been established.

~TJ
Received on Thursday, 14 January 2010 16:40:57 GMT

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