serialize a url could be clearer #6575
Description
c.f. https://drafts.csswg.org/cssom/#serialize-a-url
To serialize a URL means to create a string represented by "url(", followed by the serialization of the URL as a string, followed by ")".
This does probably mean https://drafts.csswg.org/cssom/#serialize-a-string by "as a string" (given all implementations to quote it), but we should xref this to be clear.