1) <video> and <audio> tags need volume attribute. I know that it's possible to manipulate the volume attribute with javascript. But many internet forums allow some HTML tags but prohibit javascripts.
If I was able to set the initial volume with volume attribute of <video> and <audio> tags, I would be able to set the initial volume without javascript. 2) <video> tag needs subtitle attribute. Without a subtitle attribute, people need to reencode videos with subtitles to present the videos with subtitles on the web. I think charset attribute for manually specifying subtitle's character set would come in handy also.