If you're just learning how to create your own little space in the vast expanse of the World Wide Web you are starting at a great time.
When I began learning, nearly 10 years ago, I had to crack open the books, fire up Wordpad and figure this stuff out by trial and error. If I needed help, it was a long search through various indexes.
Now, there are so many resources on the web that anyone wanting to learn HTML or any of the vast languages out there, they have gold readily available at the click of a mouse. Plus, with Web 2.0 becoming more and more prevalent, instant examples are here.
Case in point is the HTML Playground: http://htmlplayground.com/
The name says it all. A list of web coding tags are available for you to select and have instant examples and sample code to look at and then edit it and see what happens.
This is a fantastic resource for any beginner and one I highly recommend checking out and, well, playing around with!
Even though I already know all the HTML tags and their uses - I still think it is a really cool site for testing all the HTML codes. The best way to learn is just to get out there and do it. :)
After all, we already have sites for testing colors, text and other things...
I definitely agree that that is the best site for learning HTML. I just want to share some tips on good pages when you need a bit deeper knowledge on a certain HTML tag. Some good HTML reference pages are:
W3C's index of HTML4 elements
WDG's HTML 4 Element list
Glubase's wiki XHTML/HTML command reference
Theese are pretty good CSS references:
W3School's CSS2 reference
Glubase's wiki CSS2 reference