HTML Validator

Paste HTML or upload an .html file, and this validator lists every error and warning. Each one comes with its line, column, the rule that flagged it and the code around it. It runs the open-source html-validate library inside your browser, so your code is never uploaded.

html-validate is a linter-style validator. It checks your markup against the rules of the HTML standard, such as which elements may sit inside others and which attributes are required. It also runs accessibility checks and a few best-practice checks. It is not the official W3C checker, and the two do not always agree. For full conformance checking, the reference is the W3C Nu Html Checker at validator.w3.org.

Errors are real problems. Examples are an img element with no alt attribute, a p closed while a b inside it is still open, or an old tag such as center. Warnings are softer advice, such as a script with an unneeded type attribute or a button without a type. Each message links to the rule's own page, which explains why it matters and how to fix it. Copy report gives you every message as plain text.

Why fix markup that browsers display anyway? Browsers quietly guess how to repair broken HTML, and their guesses can shift layouts, break scripts or confuse screen readers. A missing alt text, for example, is both a validation error and an accessibility problem. Valid markup also makes structured data and meta tags easier for search engines to read.

The validator code, about 130 kB compressed, loads the first time you click Validate HTML. The limit is 1 MB of HTML. It checks the markup exactly as you give it, so content added later by JavaScript is not included. To show characters such as < and & as text, the HTML Entity Encoder and Decoder turns them into safe entities. The Meta Tag Generator writes valid SEO and social tags for your head section.