[css-overflow-5]: mechanism to override and set semantic props on overflow markers and buttons

[smhigley]

Some potential semantics that authors might need to add include (but are not limited to, there are probably weird edge cases I'm not thinking of):

• role
• disabled / aria-disabled
• aria-labelledby
• aria-describedby / aria-description

Using aria-labelledby could be seen as unnecessary because of the content approach, but that may not work for everyone -- often enough devs may have access to an element that should label the tab but not to the content of that element (e.g. when creating a design system, or with user-generated content). In those cases, it should be possible to name the scroll markers via reference rather than with a direct string.

The need for a description is probably a bit more self-explanatory, and isn't currently covered by CSS. Role as well -- I'm entirely sure people will use this in a way that doesn't cleanly fit the heuristics used to apply link or tab, and some accessibility SME who comes in later to fix it will need to override the semantics 😅.

One example of the need for a disabled state could be a multi-step form or process, where it's possible to navigate back through previous steps, but future steps are disabled. This would currently possible to implement for sighted users with greyed-out styles, but would need disabled semantics to be accessible for screen reader users.

[smhigley]

tagging @flackr because he made the mistake of talking to me about css overflow a few times