Over the years I've taught myself the basics of HTML and CSS through online tutorials. A quick Google search will lead you to plenty of online tutorials that you can use to teach yourself or your students some useful HTML and CSS skills. The following are the resources that I frequently recommend.
A Beginner's Guide to HTML & CSS is a nice resource developed by Shay Howe whose resume reveals that he works on the user interface for Groupon among other projects. Shay currently offers twelve text-based lessons for beginners. Once you've mastered the beginner lessons you can try your hand at the ten advanced lessons offered on the site.
Thimble is a free Mozilla product designed to help users learn how to write HTML and CSS. Thimble features a split screen on which you can write code and see how it works at the same time. On the left side of the screen you write your code and on the right side of the screen you instantly see what that code renders. If you write the code correctly, you will know right away. Likewise, if you don't write the code correctly, you will know right away. Some of the sample projects you can work with include webpages, games, and avatars.
w3Schools has long been my go-to place for quick directions when working in HTML. If I get stuck while working on a project, a quick visit to w3Schools usually reveals the help I need to get past a stumbling block. If you're completely new to writing HTML start with the introductory sections of w3Schools to learn the basics.
Once you've become familiar with the basics of HTML and CSS you may find yourself venturing into things not covered by the tutorials featured above. At that point you may want to consider joining the community at Stack Overflow to ask questions and or answer questions from other community members.