The HTML Validator checks your markup against HTML standards to catch errors, malformed tags, missing attributes, and structural problems before they reach production. Paste your HTML into the editor and the tool parses it entirely in your browser, flagging issues like unclosed elements, improperly nested tags, duplicate IDs, and deprecated attributes. Clean, valid markup renders more consistently across browsers, improves accessibility, and reduces hard-to-debug layout bugs caused by the browser silently correcting your mistakes.
HTML validation is the process of verifying that a document conforms to the syntax and grammar rules defined by the WHATWG HTML Living Standard and W3C specifications. Validation focuses on the structure of the document rather than its appearance: it confirms that elements are nested correctly, required attributes are present, and content models are respected. While browsers are forgiving and try to render even broken HTML, valid markup is more predictable, easier to maintain, and far less likely to break when you make future changes.
The main file is the entry point for each run.
Use the Input panel to pipe standard input to your program when it needs data.
Run sends your code to a fresh hosted runtime and streams the result into the output panel.
Your draft stays in this browser, and Share creates a separate snapshot link.
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>My Page</title>
</head>
<body>
<h1>Hello</h1>
<p>Valid markup.</p>
</body>
</html>
Result: No errors. Document is well-formed and conforms to HTML5.<ul>
<li>One
<li>Two</li>
</ul>
Result: Warning — the first <li> is not explicitly closed. Allowed by HTML5 parsing rules but flagged as a style/clarity issue.<img src="logo.png">
Result: Error — <img> element is missing the required "alt" attribute, which harms accessibility.<p>Text <div>block</div> more text</p>
Result: Error — a <div> (block-level) cannot appear inside a <p>; the paragraph is implicitly closed before the <div>.<div id="main"></div>
<section id="main"></section>
Result: Error — the id "main" appears twice. Each id must be unique within the document.<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
</head>
<body></body>
</html>
Result: Error — a <head> must contain exactly one non-empty <title> element.<p>Hello</span>
Result: Error — </span> has no matching open <span>; stray end tags are not allowed.<center><font size="4">Title</font></center>
Result: Error — <center> and <font> are obsolete in HTML5; use CSS (text-align, font properties) instead.<label>Email</label>
<input type="email" id="em">
Result: Warning — the <label> has no "for" attribute and no wrapped control, so it is not associated with the input.The editor uses Monaco, the engine behind VS Code, with language-aware editing tools and familiar keyboard controls.
Paste HTML into the editor
The tool parses it in-browser against HTML standards
It flags structural and syntax issues to fix before production
Unclosed elements and improperly nested tags
Duplicate IDs and missing required attributes
Deprecated attributes and malformed tags
Parsing and validation run entirely in your browser
Your markup is never uploaded or stored on a server
Valid markup renders more consistently across browsers
Reduces layout bugs caused by the browser silently fixing mistakes
No input to validate