Deprecate aria-level on listitem #1369
Description
The authoring practices task force has attempted multiple times to generate some kind of example of use of aria-level on listitem elements. We've not found a scenario that justifies doing so. In our March 31, 2020 meeting, we concluded that we should suggest removing support for it.
The scenario that came closest to seeming reasonable is when the DOM does not contain all elements of a large nested list. However, unless the DOM is going to contain invalid HTML, even this scenario does not dictate the need for aria-level. It can certainly lead to the need for aria-setsize and aria-posinset, but the structure needed to calculate aria-level would need to be present. The only possible exception would be when the DOM contains a complete list that is not nested in another list but the author wishes it to be conveyed as a nested list. Realistically, though, whatever the author does to convey that nesting to non-AT users would also need to be exposed to AT users. Additionally, the value to the end user of AT is dubious. It is more likely to be confusing to add aria-level in such a scenario.