Additional usage guidance for IDL attribute types (such as DOMString?)

[rahimabdi]

What is the issue with the HTML Standard?

The HTML spec may benefit from normative guidance on using the most appropriate type for IDL attributes.

For example, as part of a recent ARIA-motivated update to the DOMString? getter, @domenic called out that DOMString? may need to be discouraged for non-enumerated attributes. @annevk also agreed that general clarity on attribute design would be beneficial, so this issue would not be limited to DOMString? usage recommendations.