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HTML & CSS Official Documentation and Validation

W3C validators

• HTML Validator
• CSS Validator

W3C specifications:

HTML

• HTML 4.01 Specification (Final)
• HTML 5 Specification (Working Draft)
• XHTML 1.0 Specification (Final)
• XHTML 1.1 Specification (Final)

CSS

CSS 1:

• [CSS1] REC
• [CSS1 Errata]

CSS 2:

• [CSS2] REC
• [CSS2 Errata]
• [CSS2.1] CR

CSS 3:

• Introduction to CSS3 WD (Contains a table of CSS3 modules, albeit a bit outdated)
• CSS3 Color Module CR
• CSS3 Ruby Module CR
• CSS3 Text Module CR
• Selectors CR
• Media Queries CR
• CSS3 module: Basic User Interface CR
• Paged Media Properties for CSS3 CR
• CSS3 module: Backgrounds WD
• CSS3 module: Fonts WD
• CSS3 module: Web Fonts WD
  
• CSS3 Generated and Replaced Content Module WD
• CSS3 Speech Module WD
• CSS3 module: Border WD
• CSS3 module: Lists WD
• CSS3 module: The box model WD
• CSS3 module: line WD
• CSS3 module: Cascading and inheritance WD
• CSS3 module: Values and Units WD
• CSS3 module: Multi-column layout WD
• Behavioral Extensions to CSS WD
• CSS Namespace Enhancements (Proposal) WD (incorporated in Selectors)

Other CSS Specifications:

• Associating Style Sheets with XML documents
  Version 1.0 REC
• CSS Mobile Profile 1.0 CR
• CSS TV Profile 1.0 CR
• Syntax of CSS rules in HTML's "style" attribute WD

Other CSS related documents from W3C:

• XML Linking and Style NOTE
• Accessibility Features of CSS NOTE
• Using XSL and CSS together NOTE
• CSS Techniques for Web Content Accessibility Guidelines 1.0 NOTE
• Authoring Challenges for Device Independence WD