HTML Validator & Checker

Q: Is this HTML validator free?

Yes, with no account needed. The instant check has no limits at all. The W3C check uses W3C's free public service, which asks people not to send heavy automated traffic, so it's best for occasional checks by hand.

Q: Is the instant check as thorough as the W3C validator?

No, and it isn't trying to be. It catches the mistakes that cause most real bugs and explains them in plain English, instantly and privately. The W3C check tests against the whole specification. Use both: the instant check while you work, and W3C before you publish.

Q: Can I check a snippet instead of a full page?

Yes. If your code has no doctype, html, head or body tag, the instant check skips whole-page checks like the missing title. The W3C check wraps snippets in a minimal page before sending them, so you only see problems in your own code.

An HTML validator checks your markup for mistakes that browsers quietly cover up: unclosed tags, elements in the wrong place, duplicate IDs, missing alt text and obsolete code. Paste your HTML to get an instant line-by-line report right in your browser, or send it to the official W3C checker for a full standards check.

50+ checks · explained in plain English Private · instant check runs in your browser W3C · full standards check on demand

Your HTML

Click any problem to jump to its line. Ctrl + Enter re-checks. Fix it in the full editor with live preview →

How to validate HTML

Add your code. Paste HTML into the left panel, upload an .html file, or press Sample to load a page with a few mistakes in it.
Read the report. The instant check runs as you type and lists every problem with its line and column, sorted top to bottom.
Jump to each problem. Click any item to select that line in your code. Line numbers with errors are marked in red, warnings in amber.
Get the official verdict. Press Run W3C check to test the same code against the full HTML standard with W3C’s own validator.

Instant check vs. W3C check

	Instant check	W3C check
Where it runs	In your browser	On W3C’s Nu Html Checker (validator.w3.org)
Privacy	Your code never leaves your device	Your code is sent to W3C’s servers
Speed	Updates as you type	A second or two per check
What it covers	50+ of the mistakes behind most real-world bugs, explained in plain English	Every rule in the HTML standard, in W3C’s own (more technical) wording
Best for	Checking as you write	A final pass before you publish

What the instant check looks for

Structure

Unclosed and mismatched tags, stray end tags, end tags on void elements like </br>, /> on elements that can’t self-close, unclosed comments, and a missing doctype or <title>.

Nesting

Block elements inside <p>, <span> or headings, links inside links, buttons inside links, nested forms, <li> outside a list and table cells outside a row.

Attributes

Duplicate attributes, missing spaces between attributes, quotes that never close, duplicate or empty IDs, width values with units, spaces in URLs and made-up input types.

Accessibility

Images without alt text, iframes without a title, labels pointing at IDs that don’t exist, and a missing lang on <html>.

Outdated code

Obsolete elements like <center>, <font> and <marquee>, and obsolete attributes like align, bgcolor and cellpadding.

Text and metadata

Character references like &copy missing their semicolon, bare < signs in text, and a missing charset or viewport tag.

Errors, warnings and notices

Errors break the rules of the HTML standard. Browsers still try to show the page, but each one guesses at what you meant in its own way, and that’s where layout bugs, broken scripts and content that silently disappears come from. Fix these first.

Warnings are code that technically works but is outdated or likely to cause trouble later, like obsolete attributes, a missing lang or a label that doesn’t point at anything.

Notices are suggestions about style and good habits. Ignoring them won’t break anything.

Related tools and references

Frequently asked questions

Is this HTML validator free?

Yes, with no account needed. The instant check has no limits at all. The W3C check uses W3C's free public service, which asks people not to send heavy automated traffic, so it's best for occasional checks by hand.

Is the instant check as thorough as the W3C validator?

No, and it isn't trying to be. It catches the mistakes that cause most real bugs and explains them in plain English, instantly and privately. The W3C check tests against the whole specification. Use both: the instant check while you work, and W3C before you publish.

Can I check a snippet instead of a full page?

Yes. If your code has no doctype, <html>, <head> or <body>, the instant check skips whole-page checks like the missing <title>. The W3C check wraps snippets in a minimal page before sending them, so you only see problems in your own code.

Why does my page work even though it has errors?

Browsers are built to recover from broken HTML instead of refusing to show it. The catch is that every recovery is a guess. An unclosed <div> can look fine in one browser and push your footer into the sidebar in another, and screen readers and search engines can read the same mistake differently again.

Does valid HTML help SEO?

Indirectly. Google has said that passing validation isn't a ranking factor on its own. But errors that change how a page is read, like a <title> in the wrong place, a quote that never closes, or an unclosed element swallowing the content after it, can stop search engines from seeing the page the way you meant.