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CSS HTML Validator

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W3C Links

HTML 5.3 Specification

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HTML 5.1 Specification

HTML5 Specification (HTML 5.0)

HTML 4.01 Specification

XHTML™ 1.1 Recommendation

XHTML™ 1.0 Recommendation

Polyglot Markup: A robust profile of the HTML5 vocabulary

Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification

Accessibility Links

Web Content Accessibility Guidelines (WCAG) 2.0

Web Content Accessibility Guidelines 1.0

ARIA in HTML

Section 508 Accessibility Links

Section 508 Website

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HTML Tidy Links

JavaScript Lint

JavaScript Lint Home

JavaScript Lint Documentation

JavaScript Lint on SourceForge.net

JSHint

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JSHint Documentation

JSLint

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JSLint Community

Nsgmls Links

SP by James Clark

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PHP Home

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Tutorial Links

Learning HTML and CSS (with links to HTML and CSS tutorials)