Add notes for never-supported CSS types

[saschanaz]

A checklist to help your pull request get merged faster:

• Summarize your changes
• Data: link to resources that verify support information (such as browser's docs, changelogs, source control, bug trackers, and tests)
• Data: if you tested something, describe how you tested with details like browser and version
• Review the results of the linter and fix problems reported (If you need help, please ask in a comment!)
• Link to related issues or pull requests, if any: Remove items that has never been supported #6854, Remove totally unimplemented things #6768

[ddbeck]

Hi @rachelandrew, I'm going to start tagging you for review of CSS features proposed for removal, as we discussed in #6962 (comment). You don't need to do a full-review of these—I just need to whether or not they're part of an active specification. Thank you!

[rachelandrew]

@ddbeck no problem, I'll go through them.

All of the ones listed here are currently in Values and Units, or images and are legit. Some are quite new but I don't think are likely to vanish again.

[rachelandrew]

These are all part of an active spec - either Values and Units, or Images.

[ddbeck]

Thank you! 🎉