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

Shadow Roots would benefit from the UA performance features introduced in the CSS containment spec (https://drafts.csswg.org/css-containment/). Has there be consideration on how this could work well with shadow shadow?

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/355