An Advanced Guide to HTML & CSS
Published by Chris Coyier
Shay Howe has started a new learning series on HTML & CSS, this time moving beyond the basics into the real nitty-gritty stuff you need to know as a front end developer. 3/10 are done, with a new one coming each week.
If you need to start more basic, Shay's original "A Beginner’s Guide to HTML & CSS" is also great.