When setting style to a CSSStyleValue we need to convert it to a
StyleValue. If we already have one, we might as well use it avoid the
work of serialization and re-parsing.
I realised I misunderstood what "constructed from a USVString" means, so
I've adjusted based on that. It does raise a question on what the source
USVString is if that string resulted in multiple CSSStyleValues being
created - see the linked issue.