While working on the implementation in Chrome(ium), we discovered an oddity which suggests the mapping for MSAA for subscript and superscript wasn't quite right, namely: You wind up with ROLE_SYSTEM_TEXT (the subscript/superscript role) containing a child of ROLE_SYSTEM_TEXT (the static text). For example:

<h1>This text is <span role="subscript">lowered</span>.</h1>

ROLE_SYSTEM_HEADING
   ROLE_SYSTEM_TEXT "This text is "
   ROLE_SYSTEM_TEXT (subscript)
     ROLE_SYSTEM_TEXT "lowered"
   ROLE_SYSTEM_TEXT "."

Dominic Mazzoni stated that when they are used as ARIA roles they're always going to be on a grouping element and therefore wonders if the GROUPING role might make more sense.

I therefore followed up with @jcsteh. Here's what he stated:

TL;DR: There's no "good" answer for the MSAA role. I guess ROLE_SYSTEM_GROUPING will do. IA2_ROLE_TEXT_FRAME should stay as is. I think it makes sense for the HTML and Core AAMs to be consistent on this.

From what I can see, Chrome uses ROLE_SYSTEM_STATICTEXT (not ROLE_SYSTEM_TEXT) for text leaf accessibles. Even so, I'd rather we didn't use ROLE_SYSTEM_TEXT for anything other than editable text, as MSAA (kinda) suggests this is for navigable text:

The object represents selectable text that allows edits or is designated as read-only.

(Firefox still uses ROLE_SYSTEM_TEXT for text leaf accessibles, but that's somewhat of a historical wart.)

I don't really consider divs and spans to be semantic groupings; i.e. they're not semantically useful like role="group". I've always felt ROLE_SYSTEM_GROUPING was more for semantic groupings.

That said, there isn't really a generic role that fits here. ROLE_SYSTEM_STATICTEXT suggests a text leaf, which is not ideal. So I guess ROLE_SYSTEM_GROUPING will do.

At the end of the day, anyone doing anything serious with a web browser will use IA2 (not just MSAA), in which case the IA2 role overrides. So I don't really care very much about the MSAA role. :)

Therefore, we're going to make this change.