Where could I watch video on my ipad... Are there plenty of sites that support html5?

While all the major browsers, with the release of IE9 now support the majority of HTML5, it is up to the web developers to incorporate it into their websites. While adoption has been slow, I imagine it will start to pick up now as it is starting to gain some "fame" and with the new support of IE9, developers can now design their webpages in HTML5 and not break their websites for IE users, which still has the largest market share, although it is falling... if you are a web developer, you know that you make webpages for IE, while none of us like it, that is they way it is.

So with that said there is no real way to tell you how much of the internet supports HTML5. You just have to get out there and look.