Remove <frequency> and <frequency-percentage> CSS types

[ddbeck]

Summary

These types have no practical application. Let's remove them.

Test results and supporting details

Both the MDN pages for these types say they are "not currently used in any CSS properties."

This data all originated with (or was based directly on) wiki tables. The data hasn't been meaningfully updated in 7 years.

These types are effectively fictitious: the one and only place that they manifestly exist in a browser is as CSSUnitValue objects and CSS numeric static methods. There are no aural properties that exist in contemporary browsers; searching MDN and specifications yielded no evidence of proposals to use these types.

Related issues

web-platform-dx/web-features#2149 (review)

[caugner]

Good catch! Why does the linter not catch these? 🤔

[ddbeck]

Good catch! Why does the linter not catch these? 🤔

I think the impl_urls prevent this. It'd be interesting to audit features where the only data apart from version_added: false is an impl_url.