I may I sort-of praised Youtube and HTML5, allowing me to view some of the YouTube content without having Flash, and in Firefox since Google supports WebM, to some extent.

Here come the time to give some tips.

Enabling HTML5

Given the how buggy is the HTML5 implementation of YouTube, particularly with playlist and users, it is a two step process.

First, you have enable the HTML5 beta: the page will tell you the status. If it is enabled or not, what are the capabilities. If you use Firefox, you need Firefox 4 that supports the new WebM open format.

Second, to fix the UI issues, you have to use Cosmic Panda, the new UI. You enable it from that page.

At anytime you can return to these pages and revert your selection. Also you have to do that per browser - to be honest, since I'm not logged in, I can't really be sure if it sticks for the user.

Embedding

If you are embedding Youtube video with <embed>, then you are doing it wrong. This is unfortunately what a lot of plugins for CMS to. You need to use the new <iframe>. For that, when you go to the video page, click share, then embed and you'll have the snippet of HTML to paste. This will embed the video properly, using HTML5 if the viewer supports it, with the fallbacks to the usual way if needed.