Why cant I watch HD videos from youtube on my HDTV?

So I have this HDTV and its 720p I know not much right. And I have a playstation 3. I tried going on Youtube XL to watch an HD video using my PS3's web browser but the quality was 240p. Any ideas how I can fix this and watch in with a higher resolution? I think the problem is because I am using composite AV cables and not HDMI. Well thats my guess anyway. Do you think that maybe I could add &fmt=22 at the end of the url and it will be fixed?