HTML Validator

Check HTML for structural errors, unclosed tags, missing attributes, deprecated elements, and accessibility issues. Shows a collapsible tree view of the document structure.

Paste your HTML above and click Validate HTML to see the result.

About the HTML Validator

This free online HTML validator checks your markup for structural errors, accessibility issues, and HTML5 best practices — helping you write cleaner, more maintainable, and more accessible web pages.

What gets checked

Structural errors — unclosed tags, mismatched tags, unexpected closing tags, void element misuse
Accessibility — missing alt on images, missing title on iframes, missing lang on <html>
Deprecated elements — <center>, <font>, <marquee>, <frame> and others obsolete in HTML5
Obsolete attributes — e.g. align, bgcolor, border on table elements
Document structure — missing <!DOCTYPE html>, <title>, charset, viewport meta
Duplicate IDs — IDs must be unique within a document
Multiple <main> elements — only one is allowed per document
Collapsible tree view of the parsed document structure
100% client-side — your markup never leaves your browser

How to use

Paste your HTML into the input box (or click Sample to load an example).
Click Validate HTML.
Structural errors appear in the red banner with line numbers. Warnings appear in the amber banner.
Use the tree view to explore the parsed document structure.

Errors vs warnings

Errors (red) — structural problems that will cause incorrect parsing: unclosed tags, missing required attributes, void element misuse.
Warnings (amber) — best-practice violations that browsers will tolerate but may cause accessibility, SEO, or maintenance problems.