[cssom] Is LegacyNullToEmptyString allowed on CSSOMString?

[andruud]

WebIDL:

The [LegacyNullToEmptyString] extended attribute must not be associated with a type that is not DOMString. [1]

But cssom uses it in many places, and associates it with CSSOMString. Came up since Blink's IDL compiler won't accept [LegacyNullToEmptyString] on CSSOMString, which (so far) appears to be the right behavior.

[tabatkins]

I mean, clearly it's not allowed. But perhaps it should be? Or it looks like maybe we can just move it to the DOMString in the typedef?

[emilio]

I think it should be allowed on USVString at least. I opened whatwg/webidl#915 for that.

Moving to the DOMString in the typedef doesn't quite work as not all CSSOMStrings use it.