[css-will-change] 'will-change' and custom properties

https://drafts.csswg.org/css-will-change/#valdef-will-change-custom-ident
defines the behavior of 'will-change: <custom-ident>'.  It has a
number of statements that "If a[ny] non-initial value of a property
would cause"...

Xidorn pointed out to me in
https://bugzilla.mozilla.org/show_bug.cgi?id=1227501 that
technically this applies to custom properties, but it probably isn't
meant to.  I don't think implementations should be required to trace
through the uses of a custom property to see if the custom property
could possibly cause the creation of a stacking context or the
generation of a containing block for fixed-positioned elements.
That seems like a lot of work for very little benefit... and
will-change is intended to make things faster, not slower!

I think this definition should explictly exclude custom properties
from having these effects.

-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)