Previously we would just set the attributes to the serialized
descriptors, even if they were the empty string.
We now apply defaults when we have empty descriptors and apply parsing
logic from the various `set_*` methods (only applicable to `font-family`
so far where we now extract the value from either a string or a
custom-ident)
Fixes an issue in some css/css-shapes WPT tests where we weren't
properly matching fonts.
Previously we would always try to load the font URL relative to the
document's base URL. This commit means that for CSS-connected
`FontFace`s we now try to load relative to the URL of the stylesheet
that contains the associated `CSSFontFaceRule`
