Re: [webcomponents] Shadow dom: use css containment features by default (#355)

contain:style paint; might make sense for shadows as a default, but I think contain:layout is probably too restrictive as it requires you to have a definite height/width, i.e. you can't size to your children.

contain:paint is pretty restrictive, but it might be OK. It would mean that a position:absolute or position:fixed inside that container would be clipped by the container. I'm not sure how much extra optimization we could do there without contain:layout though.

We could in theory split contain:layout into two pieces. One that requires definite dimensions and another that gets the other properties (e.g. formatting context). That could be a compelling default. Then, any shadow that the author put a definite width/height on could magically get all the contain:strict benefits without the author needing to know to opt-in to contain:strict.

@tabatkins @esprehn @leviw

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