Add a property to allow exposing DOM ID

[alice]

We're working on the Servo accessibility implementation using accesskit, and we've run into a stumbling block where we seem to have no way to reliably expose the id attribute to platform APIs.

The closest seems to be author_id which is mapped to UIA_AutomationIdPropertyId in UIA and id in Atspi, but isn't mapped to AXDOMIdentifier in AX API. It may not even be appropriate to map it through to that property.

We're interested to begin using the WPT for AAMs infrastructure, which requires exposing the DOM ID as a way to find an accessibility node to make assertions about.

[mwcampbell]

I think it would be fine to map the existing author_id property to the Cocoa AXDomIdentifier attribute. Does anyone disagree?

[DataTriny]

I disagree. AXDOMIdentifier is not even documented by Apple and the entire API is marked as deprecated. We map author_id to accessibilityIdentifier because this is the up-to-date way of doing it. If we use this attribute, then we will likely have to implement all the others because this is what Core Accessibility API Mappings recommend. This document will have to be updated at some point and existing web browsers will have to switch eventually anyway.

[DataTriny]

To answer the initial question, author_id is the correct property to use to expose HTML DOM ID, Chromium does it as well.