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<_>`. Source-Repo: https://github.com/servo/servo Source-Revision: fac0d17fd6edf996876d6e6379e48ef4f9cb43d6