The current handling of custom style is far from ideal. One of the problem we have is that we rely on the validation string to do a lot of check on the input uxml/uss but we don't have any error handling if we don't have this validation string. The error would be silent and behavior, a random exception at runtime or at import time. The result of a badly formed data would be unpredictable, and not easy to diagnose.

In order to properly expose more custom style type, we would have to review a lot of the code that was build with the assumption of the validator.

This is something I personally want to expose, but it does block any workflow and we cannot introduce new features to unity 6, no matter how small they are. Even if someone add something in the next version of unity, there will be a gap before you can use it.

We are closing the bug as to be better at informing users of the things that won't happen in a predictable future.

You can suggest an entry in the roadmap at unity.com/roadmap/unity-platform/ui , and get people to vote on it. It helps focus the attention of our product owner on your wishes and need.

Thanks for your feedback! It did trigger a lot of internal discussions, even between multiples teams, that did trigger the actual reflection on where to start with the feature.