Re: [css-position-3] Rewrite Completed

> I don't recall anything else creating "fixed positioning containing block", but if there's such a thing, it'd be nice to mention it in that note as well.

Various properties other than |contain| can do that too:

  * |transform|:
    https://drafts.csswg.org/css-transforms/#containing-block-for-all-descendants
  * |translate|, |rotate|, |scale|:
    https://drafts.csswg.org/css-transforms-2/#ref-for-containing-block-for-all-descendants%E2%91%A0
  * |transform-style|:
    https://drafts.csswg.org/css-transforms-2/#ref-for-containing-block-for-all-descendants%E2%91%A2
  * |perspective|:
    https://drafts.csswg.org/css-transforms-2/#ref-for-containing-block-for-all-descendants%E2%91%A3
  * |filter|: https://drafts.fxtf.org/filter-effects/#FilterProperty
  * |will-change|:
    https://drafts.csswg.org/css-will-change/#valdef-will-change-custom-ident

The list may not be exhaustive.

-- Oriol