https://drafts.csswg.org/css-conditional-5/#style-container

Cascade-dependent keywords, such as revert and revert-layer, are invalid as values in a style feature, and cause the container style query to be false.

But my understanding is that if it is invalid, the corresponding input <style-in-parens> matches <general-enclosed> and evaluates to unknown, which is intended for an invalid feature name or value, or when any other part of the input does not match the syntax of the query/feature.

Do you want not style(--custom: revert) to evaluate to true like in Chrome, or false, which requires evaluating it to unknown?

Based on the comments in #7080, I suspect unknown is expected:

I mean that style(left: revert[-layer]) should be a parse error or "unknown" or something like that.