HTML tools
HTML Validator & Structure Checker
Run quick, pragmatic structural checks on your HTML to catch the mistakes that break a page: unclosed tags, mismatched or badly nested elements, and missing required attributes.
How to use it
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Paste or drop your HTML
Paste your HTML into the editor, or drag and drop an .html file onto the page.
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Run the checks
Click Check to scan locally for unclosed tags, broken nesting, and missing required attributes.
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Fix and re-check
Jump to each flagged issue, correct it, and run the checks again until the markup is clean.
Frequently asked questions
Is the HTML validator free?
Yes, free to use as often as you like, with no sign-up and no limits.
Is my HTML uploaded to check it?
No. Every check runs entirely in your browser. Nothing is sent to a server, so your markup never leaves your machine.
What exactly does it check?
It runs pragmatic structural checks: unclosed or mismatched tags, elements that are nested incorrectly, and missing required attributes such as
alt on images. It points you at the spot so you can fix it fast.Is this the same as the W3C validator?
No, and it does not claim to be. This is a quick local sanity check for the structural problems that most often break a page, not a full W3C conformance validation of every HTML spec rule. For a complete standards audit, run the official W3C validator as well.
Why does it flag valid-looking HTML?
The most common triggers are a tag you forgot to close, two tags closed in the wrong order, or a self-closing element written incorrectly. The message names the element and where it is, so the fix is usually quick.
My HTML is clean. How do I format or preview it?
Use the HTML Formatter to pretty-print and indent it, or the HTML Viewer to render it live in a safe sandbox and check how it looks.