If you count all of the new javascript APIs as part of HTML5, which most people do, then it is true that you can capture mic input with webRTC which is part of "HTML5".
That syntax came from MIME and has been present in email, newsgroups and the web for as long as I can remember. The server sends content-type headers delimited with MIME attributes, the browser sends accept headers and inside the pages one will often find a meta tag with its content attribute delimited that way.
Anyway this thing relies on javascript too, which I thought was a stretch to call 'pure html5'...so depending on your needs, Flash (which can use mic input) might be just as acceptable as javascript...then again, it might not be.
