[selectors-4] selector matching being specified from left to right

http://dev.w3.org/csswg/selectors-4/#evaluating-selectors introduces
an algorithm that defines how selectors are matched.  It does so by
doing the matching from left to right (across combinators), which is
the opposite of what all implementations that I'm aware of do.

Specifying it this way seems like a very bad idea, because it
introduces a number of risks:

 (1) specification authors might introduce features that are easy to
 describe in the spec's left-to-right matching but hard to implement
 in the implementations' right-to-left matching

 (2) we might want to introduce features that are simple in the
 implementations' right-to-left matching, but that are hard to
 describe it the spec's right-to-left matching

 (3) we might introduce features where it's easy to make subtle
 errors in the mapping between the two descriptions; this can lead
 to things that are technically bugs in implementations, and might
 even do so interoperably.  It's better for these not to be bugs,
 and just to match the spec.

So I think that if the spec introduces an algorithmic description of
selector matching (as the current draft of selectors-4 does), that
algorithm should work the same way implementations work.

-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)