Tutorials
Codeacademy's html and css track
HTML Tutorial on W3Schools
New features in HTML5 Tutorial at w3schools
Dive Into HTML5
Tools
Firefox's Firebug extension, and Firebug Lite for Chrome
Validation
validator.w3.org
Adobe BrowserLab
webpagetest.org
browserscope.org
quirksmode.org
Acidtest Instant Browser Tester
browserling.com For-profit service with free 5-minute sessions