seo · Free tool
Nu HTML Checker
Published by W3C · Free · Official
Validate your markup against the HTML standard — malformed HTML degrades every downstream parser of your pages.
Why it matters
Valid markup is the quiet baseline: crawlers, assistive tech and AI parsers all depend on well-formed structure.
What it helps with
- Catching unclosed tags, invalid nesting and obsolete attributes
- Keeping templates honest after refactors
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