Perhaps the most comprehensive and detailed online reference around! If you have a code-related issue, this should be your first point of reference.
One of the more comprehensive resources for code. Caveat emptor: this site tends to give you every option without parsing exactly how the code will function in a given browser.
HTML & CSS Online Courses
A really well done course presented by Shay Howe, a Chicago-based designer and educator. Check out Howe’s advanced course, too! (This one’s in development.)
A basic HTML & CSS course for non-web designers by Jessica Hische & Russ Maschmeyer. The course tackles a lot of the basics through a series of nicely produced videos, and speaks to designers on the level of design rather than strictly through code.
HTML Language Guides
The Mozilla Developer’s Network offers a fantastic set of reference tools that covers the basics of client-side web development. This is their introduction to HTML.
A great recap of Yves Peters’s presentation at the Ampersand Conference 2012 regarding web typography.
Some really nice experiments related to web typography.
A great primer on the basics of typographic form. See also their “Top 10 Dos and Don’ts of Web Typography.”
A really nice guide to better typography, presented in five steps: measurements, punctuation, ligatures, size, and weight.
If you really love typography—and I mean really love it—you will love these two sites. “I” is a blog; “We” is a gallery. Prepare to lose an hour to this site. You’re welcome.
Smashing Magazine provides a list of prominent and lovely typography blogs for your perusal.
A nice web type blog from the Type Manager for Typekit, Tim Brown.