[css-forms-1] Should `::checkmark` be allowed in appearance none?

[lukewarlow]

Related to #12365 - but more specific.

Currently we define checkmark to only exist within appearance base select options or appearance base checkbox and radio buttons.

Should we expand this to allow (require?) it in appearance none too?

It's quite common for checkboxes to use the ::before or ::after pseudo, this would give you a more "correct" one to use?

I think we should at least allow it. But I do have one reservation, why? Like does it actually provide any material benefits in the long run if we do allow this pseudo element to match in appearance none mode? Why would an author use this and not just go straight for appearance base?

[lukewarlow]

cc @nt1m because I know you previously said that appearance none ideally would get the pseudo elements.

[nt1m]

I could go either way here! There's no reason against to be honest (aside from maybe consistency across all form controls)