Carryover evaluation: Containment

[foolip]

Let's evaluate whether to continue the Containment focus area in Interop 2024.

Original proposals:

• Containment #19 by @una
• css-contain-2 and contain-intrinsic-size #189 by @vmpstr

Tests:

• All tests
• Failing tests

We should also consider whether we should add tests for 2024.

[foolip]

Disposition of remaining failures:

• Chrome failures were fixed by @danielsakhapov in https://crbug.com/990657 and should be visible on wpt.fyi soon
• Firefox failures are all linked to specific bugs. I think that https://bugzil.la/1660384 is the biggest chunk, but listing all bugs here doesn't seem useful.
• Safari failure of contain-intrinsic-size-030.html failure seems to be because of scrollbar-gutter support. I linked the bug. The majority of the remaining failures seem to be about quotes and counter containment.