A couple of issues here:

serialize a CSSVariableReferenceValue step 3 says:

3. If this’s fallback internal slot is not null, append ", " to s, then serialize the fallback internal slot and append it to s.

However, the tested behaviour in https://wpt.live/css/css-typed-om/stylevalue-serialization/cssUnparsedValue.html requires that "," without a space is appended instead.

serialize a CSSUnparsedValue somehow needs to obey the comment-insertion rules as described here. This is awkward because we have a list of strings and CSSVariableReferenceValues, not CSS tokens.