Yeah just the other day I realised this doesn't work (looking up syntax did not tell me anything, because they did not actually post real syntax, only an example of code... this was the one time where I would have rather seen syntax):
This'll create "header" but not the others. No, each has to be listed separately.
Then you have to display: block them.
Then you have something. Something browsers still see as spans styled like blocks. The only software I know of who's even started seeing any meaning in the new tags yet are some screen readers, and they're still working on that (basically, a "nav" element will get an intrinsic role of "navigation" so the screen reader is reading the browser's output to AT when the browser tells the AT about the element.
Steve Faulkner recently posted a screen shot of some info given to AT about an element (this is an example of a proposed new element, <main>):