Just addressed all issues, PTAL

Thanks! I refactored this slightly to handle:

• Using a custom type for declarations so they are fully parsed. With DeclarationBlock they'd all be parsed as unknown properties, and things like !important shouldn't be allowed.
• Removed the font-display since it is not supported by any browser and will be removed from the spec.
• Refactored FontFamily to separate FamilyName from generic family parsing. Generic families shouldn't be allowed here.
• Handled merging duplicate declarations and rules.