One of the valuable parts of the mdn-data package is how it separates CSS features into different categories:

• At-rules
• functions
• properties
• selectors
• syntaxes
• types
• units

In the current webref package, it's just a collection of objects that we then need to dig into to figure out what types are contained within. It would be helpful if the categories could be exposed at the top level of the package and list every entry for that category regardless of spec.