We don't want UAs to cut out functionality. Issue of a built-in speech engine interfering with the user's speech engine. Propose text for the techniques document about synthesized speech implementation issues. Need to require override of author-specified speeds. Speech engines do whatever they're told to do in the source.
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.