This includes whatever granularity offered by speech engine. Font requirement implies big performance hit, reflow. Even with bold, there may be reformatting. I have concerns about the font requirement as well. Opera offers border as highlight mechanism.
Color suffices for many users with low vision.
It's more critical than text style, and in many cases, changing the font style is disorienting. So the final formatting depends on that priority. There's another piece to this - when you're actually selecting something, it concurrently has selection and focus. Haven't asked RS yet. PF people didn't know of any implementations.