Thimble won't teach you HTML directly—you'd still want to have something like W3Schools or Code Academy on hand—but it's a tool that makes learning a lot easier. Instead of having to type your code in a text editor and load the resulting file up in a browser, you can see all your changes in real time as you make them. That also makes correcting mistakes a lot easier, since you'll know exactly what caused a problem as soon as you type it. You can even publish your finished web pages right from Thimble's app for all your friends to see. It's still in the testing phases right now, but you can use the testing version by clicking the link below.
Mozilla Thimble | via Webmonkey