Consider requiring user agents to calculate aria-level, aria-posinset, and aria-setsize when not declared by authors

[mcking65]

The proposed comment role includes the following language:

If the author has not explicitly declared aria-level, aria-posinset, or aria-setsize for a comment element, user agents MUST automatically compute the missing values and expose them to assistive technologies.

Currently, ARIA does not include a normative requirement for computing these properties when they are used on elements with other roles that support them. For example, the definition of aria-level includes the following language:

If the DOM ancestry accurately represents the level, the user agent can calculate the level of an item from the document structure. This attribute can be used to provide an explicit indication of the level when that is not possible to calculate from the document structure or the aria-owns attribute. User agent support for automatic calculation of level may vary; authors SHOULD test with user agents and assistive technologies to determine whether this attribute is needed. If the author intends for the user agent to calculate the level, the author SHOULD omit this attribute.

So, user agents are explicitly given a pass here. Let's consider changing this language to require calculation.