Check out my HTML5 Coders Bookshelf (coming soon) if you'd like to learn by the old-school reading-a-book method.
The HyperText Markup Language was developed primarily over a ten year span from 1989 to 1999. In 1999, coders settled out with a tweak to end up with HTML 4.01.
One problem with HTML 4.01 was that it had a really long, difficult to memorize opening DOCTYPE. If you've seen the movie Office Space, this might well relate well to the "TPS Report." DOCTYPEs are great, in theory, but the one for HTML4.01 (and XHTML later on) were, in my opinion, a huge excessive waste of bandwidth. Read about the new HTML5 DOCTYPE (link coming soon), which is a lot easier to use, and much more coder-friendly.
I've been coding HTML for nearly eighteen years now, and the basic knowledge I learned from online tutorials back then, eighteen years ago, about HTML still apply today.
A web page that was coded with the HTML versions used ten, fifteen, or even twenty years ago still work in almost all modern web browsers (even that brand new Chrome, Firefox, or Safari update).
In my estimation, it's likely that HTML5, and its enhancements, will be used until 2025 at least.
(My true hope is that we can publish the research on the cure to cancer in HTML5.)
There are already a wealth of tutorials, guides, and even apps on web design, a lot specifically on HTML5. If you don't know HTML at all yet, check out Web Design Basics for iPad.
I'm spending my time right now focusing on Web Design Basics 2 (it doesn't have an official title) which will be the bridge the gap between The Basics (in my current app) and HTML5.
There's a lot to learn between your first few lines of HTML and CSS and understanding HTML5, so I'm hoping between my two courses, Web Design Basics and Web Design Basics 2, that you'll be able to start your journey in HTML5.
At this point, I'd suggest three things: