Validate HTML online. Check for missing attributes, unclosed tags, deprecated elements, and accessibility issues. Free, instant, runs in your browser.
HTML validation checks your markup against web standards, accessibility guidelines, and browser compatibility rules. Catching issues early — missing alt attributes, unclosed tags, deprecated elements — prevents subtle bugs that are hard to diagnose in production.
This validator checks for: missing DOCTYPE declarations, absent <meta charset> and viewport tags, missing <title>, unclosed tags, images without alt attributes, deprecated elements, and common accessibility issues. It does not validate against the full W3C HTML spec — for that, use the official W3C Markup Validation Service.
Browsers are extremely tolerant and will silently correct most HTML errors using error recovery algorithms. Passing a browser rendering test does not mean your HTML is valid. Invalid HTML can cause inconsistent behaviour across different browsers, break screen readers, and produce unexpected results with CSS selectors.
Elements like <font>, <center>, <marquee>, and <frameset> were removed from the HTML specification. While browsers still support them for legacy reasons, they should be replaced with CSS equivalents: font-family/color, text-align: center, CSS animations, and <iframe> respectively.
No. All validation runs entirely in your browser using JavaScript. Your HTML never leaves your machine — important for templates containing internal URLs, API endpoints, or sensitive data.