XML and HTML don’t actually have a lot to do with other in practice.
You can write HTML with an XML syntax, but when you’re using HTML to create web-based apps, most of your work will be:
- figuring out what HTML elements do
- figuring out how to make them look nice (via CSS) and provide more user interactions than they do by default (using JavaScript)
- figuring out how they work in different browsers
None of this has anything much to do with XML.
For starting out with HTML, you might want to try O’Reilly’s HTML: The Definitive Guide (5th edition). It doesn’t cover HTML5 yet, but it’ll give you a solid grounding.
Then, as already mentioned, Dive into HTML5 is great for learning the new stuff in HTML5. It assumes you already know some HTML, which is why I’d suggest the O’Reilly book or similar first.