[selectors4] Not clear where pseudo-elements fit in

I was trying to read selectors4 to determine whether a
pseudo-element inside of :host-context() [1] made the selector
invalid (or just never matched... I think).

I reached http://dev.w3.org/csswg/selectors-4/#structure , and found
that compound selectors do not include pseudo-elements, which I
believe answers my question to my satisfaction (and matches Gecko's
current implementation of :-moz-any()).

However, I then noticed that nothing in that section mentions
pseudo-elements at all, so it's not clear that my interpretation is
correct.

http://dev.w3.org/csswg/selectors-3/#selector-syntax , on the other
hand, does say that pseudo-elements fit in at the "complex selector"
level (level 3's "selector"), which makes it more clear that they're
intentionally not included as part of a "compound selector".

I think it would probably be good to have similar wording, and maybe
even something clearer, in Level 4, so that it's clear that
pseudo-elements are not part of a "compound selector" and thus their
presence in a <compound-selector> [2] is a parse error.

(Unless, of course, the opposite is intended, in which case that
should be clear.)

-David

[1] http://dev.w3.org/csswg/css-scoping/#selectordef-host-context
    which links to [2]
[2] http://dev.w3.org/csswg/selectors-4/#typedef-compound-selector

-- 
𝄞   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)