Proposal for limited :matches pseudoclass

This is lifted from the :has-child proposal thread, since at least one
person (Brad Kemper) liked the syntax, and at least one implementor (Boris
Zbarsky) thought it wouldn't be horrible for implementations (at least, no
more horrible than current child and sibling combinators).  Thought I'd give
it a proper write-up, since I've had reason to want it in the last week.

*:matches pseudo-class*

The :matches(*selector fragment*) pseudo-class represents an element that
has another specified element *after *it in the document tree.  The selector
fragment starts with either the child combinator (>) or the adjacent-sibling
combinator (+), optionally followed by whitespace, followed by a simple
selector (see Selectors Level 3 for the definition of a simple selector).
This matches if an element matched by the simple selector occurs as a child
(if > was used) or following adjacent sibling (if + was used) of the target
element.  If the selector fragment is invalid or empty, the entire selector
is invalid.

Examples:
  td:matches(> input:checked) /* Matches any td with a checked input as a
direct child */
  h1:matches(+ ul) /* Matches any h1 which is directly followed by a <ul> */
  p:matches(+ p) /* In a string of sibling <p>s, matches all but the last.
Contrast with "p+p", which matches all but the first. */

  tr:matches( input:checked) /* Invalid, as descendant combinator is not
allowed */
  div:matches(> p > samp) /* Invalid, as combinator is not followed by a
simple selector */

Future extensions may relax the restrictions on the selector fragment, such
as allowing it to take more combinators (such as the descendant or general
sibling combinators) or allowing more than a simple selector in the selector
fragment.

~TJ