[css-ui] formality of selector definitions

I think the selector definitions in
http://dev.w3.org/csswg/css-ui/#user-interface-selectors are too
informal and leave too much to user agents.  It's unclear what in
their definition constitutes a conformance requirement (and whether
that requirement is on implementations or specifications).

In order to reach interoperability, we want the semantics of these
selectors to match in the same states for a given markup language.
If we don't produce that level of interoperability, eventually
either (a) we'll be forced to as a result of author complaints about
lack of that interoperability or (b) the technology will fade away
due to lack of use.

I think many of these definitions should essentially be separated
into two parts:

 (1) an implementation conformance statement that the selector
 matches when (and only when) the specification defining the markup
 language says it it does

 (2) guidance for designers of languages on what the selector is
 designed for

I think it would also be useful to point informatively to the
definitions in the HTML specification, since that's useful for
authors:
http://www.whatwg.org/specs/web-apps/current-work/multipage/selectors.html#pseudo-classes

-David

-- 
𝄞   L. David Baron                         http://dbaron.org/   𝄂
𝄢   Mozilla                          https://www.mozilla.org/   𝄂
             Before I built a wall I'd ask to know
             What I was walling in or walling out,
             And to whom I was like to give offense.
               - Robert Frost, Mending Wall (1914)