Moving this issue from csswg issue #12561.

Step 2.1. of serialize a CSSUnparsedValue appends USVStrings without any separation.

1. Let s initially be the empty string.
2. For each item in this’s [[tokens]] internal slot:
   1. If item is a USVString, append it to s.
   2. Otherwise, item is a CSSVariableReferenceValue. Serialize it, then append the result to s.
3. Return s.

Implementations differ here.

• Blink append /**/ between subsequent USVStrings
• WebKit does not append anything between subsequent USVStrings
• WPT expects /**/