Code Race is a website that pits you against other coders in a race to solve HTML and CSS problems.
To be able to use it you need to already have a good understanding of HTML and CSS – it would be too difficult for beginners. However the concept is quite fun and certainly gets you using HTML quickly and accurately.
This website comes from the team behind the online code learning site www.teamtreehouse.com. Their website is sadly subscription only for the majority of their tutorials, but their HTML content is free and a good resource for classes.