Bug 238273

In https://github.com/w3c/csswg-drafts/issues/5649#issuecomment-755796005 the CSS WG resolved to split CSSStyleDeclaration into smaller interfaces, splitting up the current mix of properties and descriptors that WebKit and Blink have. In short, we should end up interface CSSStyleProperties : CSSStyleDeclaration {} containing attributes for all supported CSS properties (but nothing that's only a descriptor), along with many interfaces like interface CSSFontFaceDescriptors : CSSStyleDeclaration {} containing attributes for all the descriptors that apply to @font-face rules, CSSCounterStyleDescriptors containing attributes for all the descriptors that apply to @counter-style rules, etc. At the moment, we don't know what properties/descriptors apply to what, besides knowing those annotated with "descriptor-only": true aren't properties. We probably need to start off by introducing some knowledge of what applies to what (e.g., by relating each thing in CSSProperties.json to StyleRuleType).