HTML Validator
Find malformed tags and attributes, mismatched or unclosed elements, common nesting problems, duplicate IDs and missing image alt text—with line and column locations.
| Level | Location | Check | Diagnostic |
|---|
Common HTML source and structure issues
Document mode checks the page shell; Fragment mode is useful for snippets and components.
Tag structureUnexpected or mismatched closing tags, unclosed non-optional elements, malformed tag endings and unclosed comments.AttributesRepeated attributes, invalid attribute syntax and unquoted values containing characters that require quotes.Common nestingBasic parent rules for list items, table rows/cells, options and figure captions; common optional end tags are accounted for.Accessibility hintsMissing image alt attributes, duplicate or empty IDs, and a missing document language attribute.A useful source checker—not a full conformance oracle
This client-side checker tokenizes source text without creating a live DOM. It does not execute scripts, fetch images/stylesheets, or send HTML to a server. Script and style contents are treated as text so apparent tags inside code are not reported as markup.
It understands common optional HTML end tags such as <li>, <p>, <option> and table cells, and recognizes self-closing syntax inside SVG/MathML.
The checker covers syntax, structure and selected common issues, but it does not implement every WHATWG parsing/conformance rule, every element content model, every attribute rule, CSS/JavaScript syntax, or browser rendering behavior. Warnings are review prompts, not proof that a page is broken.
Input is limited to 1 MiB, nesting to 256 elements, markup tokens to 200,000 and displayed diagnostics to 2,000. Use an official full HTML conformance checker when exhaustive standards validation is required.
Frequently Asked Questions
No. It checks source structure and selected common issues, but not every WHATWG parsing rule, attribute value, element content model or rendering behavior. Use a full conformance checker when you need exhaustive validation.
Document mode checks for a doctype, <html>, <head>, <body> and a title. Fragment mode skips whole-document checks for components and snippets, while still reporting tag and nesting issues.
No. HTML allows some end tags to be omitted. The checker accounts for common cases such as list items, paragraphs, options and table cells. Other unclosed elements are reported.
No. HTML is scanned as text in a local worker. It is never rendered, scripts are never executed, and links or resources are not fetched.
No. Validation runs in your browser and the tool does not store your input.