Html Validation Test
The HTML Validation Test parses any URL through a strict W3C-compliant HTML5 validator and reports every error, warning and quirk. While modern browsers tolerate broken HTML gracefully, parser errors can cause Googlebot to misinterpret your structure, accessibility tools to misread the page, and edge cases (AMP, RSS, structured data) to silently fail. Clean, valid HTML is the foundation every other SEO and accessibility signal sits on.
What This Tool Checks
- Unclosed or improperly nested tags
- Deprecated HTML4 elements still in use
- Invalid attribute values
- Duplicate id attributes
- Missing required attributes (alt on img, etc.)
- Mismatched <head> / <body> structure
- Mismatched HTML doctype declaration
Why It Matters for SEO
Browsers paper over broken HTML, but every other tool that reads your HTML — Googlebot, accessibility tools, structured-data validators, RSS / Atom feeds, AMP — interprets the spec more literally. Pages with valid HTML behave consistently across crawlers and tools; pages with broken HTML produce silent failures that are hard to debug after the fact. Validating HTML is cheap insurance against weird-edge-case bugs.
How to Fix It
Fix each error reported here in order of severity. CMS-driven pages may need template fixes rather than per-page edits. Add HTML validation to your CI / build pipeline so regressions are caught at deploy time. Prefer linters (htmlhint, htmlvalidate) integrated into your editor for live feedback.
How It Works
We pass the page HTML through a strict W3C-compliant parser, collect every spec violation with line and column numbers, and group by severity. Each error includes the exact remediation needed.
Common Mistakes to Avoid
- Unclosed <p> tags from CMS import
- Duplicate id attributes generated by repeated component templates
- Deprecated <font>, <center>, <marquee> from legacy markup
- Invalid attribute values like loading="lazi" (typo)
- Self-closing tags on non-void elements (<div />)
Quick Checklist
- Zero validation errors on production HTML
- No duplicate id attributes
- No deprecated HTML4 elements
- All required attributes present (alt, lang, etc.)
- Validation step in CI pipeline