Learning HTML5

For a diversion, and for my own edification, I'm learning HTML 5.  I used to have a very academic understanding of HTML (4 and 4.01) and was very familiar with DFDs and the spec.  It's been a long time since then, and I need to learn CSS and javascript as well as the new HTML5 capabilities.

For starters I'm using material available on w3schools.com which I've found in the past to be a good teaching website - if again slightly academic in its slant.

Whither do you put javascript?

That's the first big thing I'm finding.
The sample on HTML5 drag and drop puts the javascript in the document's head section - and it works fine.  But the sample on HTML5 video DOM puts the javascript in the document's body - and it wont work if the javascript is in the header.   Why is that?