Remove never-supported CSS font-variant subfeatures

[saschanaz]

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• Link to related issues or pull requests, if any: Remove items that has never been supported #6854, Remove totally unimplemented things #6768

[rachelandrew]

Uppercase_eszett should stay, it's in a spec at CR.

The Greek accented chars isn't a value. This was added by @ddbeck so maybe he has the history #4608

[rachelandrew]

Actually, I think I'm wrong there, I think these two things should be treated in the same way @ddbeck?

[ddbeck]

Yeah, let's leave these intact for now. #4548 is probably interesting reading too. There's a compatability consequence for web developers to know that some systems may do the correct behavior, even if the browser hasn't implemented it in a consistent way. This was super weird to research in the first place and I feel kinda weird keeping it, but I don't have a better idea at the moment.

Thank you @rachelandrew and @saschanaz!