[csswg-drafts] Pull Request: [css-containment] Clarify ambiguous phrasing

frivoal has just submitted a new pull request for 
https://github.com/w3c/csswg-drafts:

== [css-containment] Clarify ambiguous phrasing ==
In a few places, this is an editorial change merely making sure
that there is no way to misinterpret what was meant.
In addition this gives a normative definition
to the previously vague requirement that
in certain cases an element "must be a formatting context".

Closes #830

See https://github.com/w3c/csswg-drafts/pull/831