Validate HTML against W3C standards

Invalid HTML causes unpredictable rendering across browsers, breaks accessibility tools, and makes debugging significantly harder.

Quick Reference

• Use validator.w3.org or browser extensions for validation
• Fix errors first, then warnings (errors cause rendering issues)
• Integrate validation into CI/CD with html-validate
• Common issues: unclosed tags, invalid nesting, missing attributes

Check

Validate HTML markup using the W3C HTML Validator to identify and fix markup errors that could cause cross-browser compatibility issues.

Fix

Run HTML through W3C validator and fix reported errors including unclosed tags, missing attributes, and invalid nesting.

Explain

Explain why W3C HTML validation is important for cross-browser compatibility, accessibility, and maintainable code.

Code Review

Review templates, server-rendered HTML, and shared components that output markup related to Validate HTML against W3C standards. Flag exact elements, attributes, and routes where the rendered HTML violates the rule.

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For full implementation details, code examples, and framework-specific guidance, see references/rule.md.

Rule page: https://frontendchecklist.io/en/rules/html/w3c-compliant