Exposure of @container in CSSOM

[gsnedders]

CSS Contain 3 defines an at-rule: https://www.w3.org/TR/css-contain-3/#container-rule

It doesn't, however, define how this gets exposed via the CSSOM, which I would expect to be specified. Should it just be exposed as an (apparently abstract) CSSRule, or should it have its own interface? If it does, should it be added as a value type can return: https://www.w3.org/TR/cssom/#dom-cssrule-type?

[mirisuzanne]

This is in the Editor's Draft, and just needs to be published