It'd be nice to get this fixed, there are WPT tests relying on the incorrect Blink behavior which I'm fixing in https://bugzilla.mozilla.org/show_bug.cgi?id=1425227.

So I have a very rough WIP patch for this, but the main problem I ran into was for dashed attributes. Our current IDL parser doesn't allow dashes anywhere in an attribute name (contrary to the spec, which allows dashes anywhere other than the first character), meaning that attributes like font-size don't work. But even if that is fixed, WebIDL doesn't allow dashes as the first character of an identifier, meaning that -webkit/other prefixed properties cannot be attributes in WebIDL.

Firefox avoids this issue as they have implemented a special extended attribute of [BindingAlias=] (see https://searchfox.org/mozilla-central/source/dom/bindings/GenerateCSS2PropertiesWebIDL.py#58) which means that they only define one attribute (the lower camel case one) for each property and the rest are aliased to that, without needing to be valid WebIDL identifiers.