Bug
1896047 part 5: Support CSS alt text for text content. r=eeejay
CSS alt text makes most sense for an image.
However, even when the content property specifies text, you can still specify alt text!
To support this, this patch does two things:
1. Uses the alt text for the TextLeafAccessible instead of the primary text.
2. Tweaks nsTextEquivUtils to use the alt text when gathering text from a subtree.
As with the last patch, changing the CSS content replaces the pseudo-element, which in turn replaces the Accessible with a new one, so we don't need any additional code to handle updates.
Differential Revision: https://phabricator.services.mozilla.com/
D210017