While there are more guides to HTML and CSS on the web than you can throw a stick at, few of them have taken into account the new-fangled tools designers now take for granted such as jQuery, CSS3 and other HTML5 goodness. In this respect, The Advanced Guide to HTML and CSS is a godsend. While it’s being released in weekly segments, it’s free.
On a more prosaic note, how do you do blockquotes and citations in HTML5? The guys over at A List Apart have thought this through (tl;dr: use <figure> as a container, then <blockquote> and <figcaption> with cite for the title).
I’ve had my own experiments with working with HTML5 by hand (somewhat), see my notes on making a handcoded one-page responsive site on my ‘proper’ site.
I incorporated a few elements from the Twitter Bootstrap framework in the site, but had I known about Bootsnip I’d have pulled bits from that.
Speaking of nice HTML: Minimal Monkey has some lovely code to investigate.
[EDIT: This post of HTML polyfills is pretty useful should you need to account for older browsers *cough* IE6 *cough*]