Fix#16165.
Also, it turns out that the CSSFontFaceRule IDL specified in the
css-fonts spec is not web-compatible.
Instead browsers implement a .style attribute like in CSSStyleRule:
https://github.com/w3c/csswg-drafts/issues/825
This in turn requires preserving data about which descriptors
were set or not (distinguishing unset from set to a value that happens
to be the initial value),
so this commit also makes every field `Option<_>`.