Learn

General

• HTML 4.01 Tutorial (May 2005)
• Advanced HTML 4.01 Tutorial (September 2008)
• Adding a touch of style (March 2009)
• Style Guide for online hypertext
• W3C I18N Techniques: Authoring (X)HTML & CSS
• HTML Tidy

The Language

• List of HTML and related elements

Learning HTML5

• Dive Into HTML5 by Mark Pilgrim (June 2010)
• HTML 5 and CSS 3: The Techniques You’ll Soon Be Using (July 2009)
• Coding A HTML 5 Layout From Scratch (April 2009)
• HTML5 articles on A List apart
• Canvas tutorial at Mozilla
• HTML 5 canvas - the basics at Opera
• Game Development with JavaScript and the Canvas element
• See also the list of new HTML5 elements for quick examples

Validators

• html5 Conformance Checker (alpha)
• W3C Validation service