Hi Osa,
NetTalk already uses a lot of HTMl5 and CSS3.
Bear in mind that neither are "complete" yet - the standard hasn't even been finalized, and some browsers support what there is of the specification more or less than others. For example in the File Uploading you don't get a progress bar in IE9, but you do get one in IE10. That sort of thing. there are lots of HTML 5 feature support in NetTalk - it's mostly invisible to you.
so perhaps the real question is - which feature of HTML5 were you hoping to use?
cheers
Bruce