It would be great to have an alphabetical list of the permitted tags, each one expandable so users don't have to scroll through the list. The alphabetical list would help semi-experienced HTML users who see the list of permitted tags and just want to look up that one tag they don't know.

But for most users, a grouped list will be much more useful. I suggest the following groups as useful for beginners: "Formatting", "Forms", "Images", "Links", "Lists", "Tables", "Other".