[css-typed-om] Unclear how to get the source string when multiple CSSStyleValues are created from USVString

[AtkinsSJ]

The serialization for CSSStyleValue says:

if the value was constructed from a USVString
the serialization is the USVString from which the value was constructed.

The trouble is, that a single USVString can produce multiple CSSStyleValues. As far as I can tell, all paths go through "parse a CSSStyleValue", which parses the string as a list of CSSStyleValues, optionally returning just the first one. The source string then wouldn't correspond to one CSSStyleValue, but only part of it would.

For example:

// parse() means we only get the first value
var cool = CSSStringValue.parse("background", "url('cool.png'), url('rad.png')");

// Source string is "url('cool.png'), url('rad.png')" but this should only log "url('cool.png')".
console.log(cool.toString());