Setting the DOCTYPE
HTML 5, when used as plain HTML rather than its XHTML 5 sibling doesn’t need a DOCTYPE. But all browsers do, otherwise they go into Quirksmode, which you don’t want: the collision of HTML 5 and Quirksmode is like matter and anti-matter meeting, and will cause a negative reality inversion that will make your underwear catch fire.
You’ve been warned, so at the top of your document, you need the line <!DOCTYPE html>.
Some sites “use” HTML 5, when in actual fact all they’ve done is take their existing code and change the DOCTYPE. That’s fine and dandy if you’ve been using valid, semantic code as HTML 5 is very similar to valid HTML 4.01. Eric Meyer mentions small differences like “not permitting a value attribute on an image submit”, and there are a few differences between the languages, summarised in the document HTML 5 differences from HTML 4.
However, I don’t want simply to rebadge my existing code, but to use some of the new structural elements now.