aria-level for headings (AXAPI)

[rahimabdi]

I noticed the aria-level core-aam mapping does not specify that a special value of aria-level="0" is allowed for headings in WebKit which conveys this as "heading" without a level: https://commits.webkit.org/271497@main.

I was curious if additional clarification is needed regarding this; for example in the group name section, spec states the following:

aria-level, aria-posinset, and aria-setsize are all 1-based. When the property is not present or is "0", it indicates the property is not computed or not supported.

This isn't entirely true as WebKit supports it (although "computed" may have a different meaning than I understand in this context). Do other UAs also treat 0 as a special case?

Note: there is an existing PR from @scottaohara addressing several related posinset/setsize concerns: w3c/aria#2341