[css-contain-3] Size container features should not be formatted as a CSS descriptor

[pyoor]

Please correct me if I'm wrong, but a CSS descriptor should only occur within the body of a CSS at-rule and exist in the format of name: value. In the CSS Contain specification (and Media Queries), the <size-feature> CSS type is described using the descdef class which should be reserved for descriptors. This breaks machine reading of the spec (i.e. webref. I'd recommend that the size-feature definitions are formatted similar to any other CSS type syntax.

[dbaron]

I think the formatting of mediaqueries-4's width and of css-contain-3's width is useful. It's not clear to me that we need a class separate from descdef for this, but if we do, it seems useful to retain the current formatting.

Being a descriptor definition, on its own, isn't that useful, because you need to know what that descriptor is for. We probably ought to mark that up. And if we're doing that, it seems like that markup could express whether it's for the body of an @-rule or for the prelude of a grouping @-rule.

[pyoor]

My only argument for a new format, or to modify the existing format, would be to match that of other specifications. For instance, the @page and @import rules both include the prelude in the same manner that general CSS syntax is defined with additional information defined in the prose.