[css-transforms][css-filters] concept of establishing containing block

In https://drafts.csswg.org/css-transforms/ , the 'transform',
'transform-style', and 'perspective' properties, and in
https://drafts.fxtf.org/filters/ the 'filter' property [1], are
defined to establish a containing block for fixed-positioned
elements.  I think many people have assumed this to be the case, but
it should be explicitly stated, that these also establish a
containing block for absolutely-positioned elements.

https://drafts.csswg.org/css-containment/#containment-paint
specifies this correctly.

-David

[1] as resolved in
    http://www.w3.org/2015/02/10-fx-minutes.html#action02 but not
    yet edited, although it is mentioned earlier in the spec.

-- 
𝄞   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)