Validate HTML against W3C standards
SkillWeb & browsingW3c-compliant is an agent skill for validating HTML against W3C standards. It guides the agent to check the final browser-facing markup, not just the source framework abstraction, and to fix errors such as unclosed tags, invalid nesting, and missing attributes. This helps prevent unpredictable rendering across browsers and keeps pages accessible.
Available today. Use it from your connected AI after setup.
No other account needed.
Have an agent that can load skills, and the skill files available to it.
Then ask your AI: use the Validate HTML against W3C standards skill
What your AI can do with it
- Validate HTML markup using the W3C HTML Validator
- Fix reported errors including unclosed tags, missing attributes, and invalid nesting
- Prioritize fixing errors before warnings, since errors cause rendering issues
- Review templates, server-rendered HTML, and shared components for rule violations
- Explain why W3C HTML validation matters for compatibility, accessibility, and maintainabil
- Integrate validation into CI/CD with html-validate
Getting started
- Have an agent that can load skills, and the skill files available to it.
- Add the skill to the agent's skill list so it can be triggered when reviewing templates or rendered HTML.
- Ask the agent to review a template, rendered page, or shared component for W3C compliance.
- Apply the fixes the agent reports, such as closing tags, adding missing attributes, and correcting nesting.
- Optionally set up html-validate in CI/CD so validation runs automatically.
What this skill tells your AI
The instructions your AI receives, as published by thedaviddias/front-end-checklist in skills/w3c-compliant/SKILL.md and read by ahel’s review.
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.
For full implementation details, code examples, and framework-specific guidance,
see references/rule.md.
Rule page: https://frontendchecklist.io/en/rules/html/w3c-compliant
Signals
- GitHub stars
- 74k
- Forks
- 7k
- Last commit
- Aug 2026
ahel review
K1binfo
installs-packages (in references/rule.md)
Automated review, not a security audit. Ruleset v1+k2.
Questions
- What does the skill validate?
- It validates the final browser-facing HTML against W3C standards, not just the source framework abstraction, using the W3C HTML Validator.
- What common issues does it catch?
- Unclosed tags, invalid nesting, and missing attributes are listed as common issues.
- Should errors or warnings be fixed first?
- Fix errors first, then warnings, because errors cause rendering issues.
Advanced
- Item type
- skill
- Key
w3c-compliant- Source
- github.com/thedaviddias/front-end-checklist