[css-will-change] establishing containing block for fixed-positioned elements

https://drafts.csswg.org/css-will-change/#valdef-will-change-custom-ident
says:

  If any non-initial value of a property would cause the element to
  generate a containing block for fixed-position elements,
  specifying that property in will-change must cause the element to
  generate a containing block for fixed-position elements.

I think this should instead say:

  If any non-initial value of a property would cause the element to
  generate a containing block for fixed-position elements,
  specifying that property in will-change must cause the element to
  generate a containing block for fixed-position _and
  absolute-position_ elements.

I don't think we need special will-change handling for the
properties that establish a containing block for absolute-position
but not fixed-position elements (i.e., the position property), but
the properties that establish a containing block for
fixed-positioned elements *also* do so for absolutely-positioned
elements.  And it would be bizarre (and defeat the point of the
special will-change handling) to establish only half of the
containing-block nature and not all of it.

(Also see
https://lists.w3.org/Archives/Public/public-fx/2015OctDec/0035.html
about making this clearer in css-transforms and css-filters.)


While here, it's probably also worth using "absolutely positioned"
and "fixed positioned" as
https://drafts.csswg.org/css-containment/#containment-paint does,
rather than "fixed-position" and "absolute-position".

-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)