HTML has been on a wild ride. Sure, HTML started as a mere markup language, but more recently HTML’s put on some major muscle. Now we’ve got a language tuned for building true web applications with local storage, 2D drawing, offline support, sockets and threads, and more.
If you’ve never had exposure to HTML5 before, that’s okay, but you should have worked with HTML, and there are some basics you should know about like elements, tags, attributes, nesting, the difference between semantic markup and adding style, and so on.
If you aren’t familiar with all these, we’re going to make a small suggestion (and a shameless plug): there’s a book called Head First HTML
with CSS & XHTML, and you should read it. And if you’re somewhat familar with markup languages, you might want to skim it or use it as a reference while coding HTML5.