[CSS] `display: contents` behavior

[pkra]

Just another idea for css-aam. It could help reduce developer confusion to document something here.

[pkra]

For a example, a dialog element with display:contents behaves a lot like one with the open attribute (at least non-visually).

[rahimabdi]

Thanks @pkra! I think most of the display: contents browser bugs have been resolved, although there are outstanding bugs (and questions) around focusability of controls. Here's the related interop issue: web-platform-tests/interop#568.