Some control sequences hard-coded in content to control a particular speech engine. That's like painting to the screen - can't be interrupted on the fly. More advanced user agents let you specify different rates for different windows, objects, content. No change to checkpoint. 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.