Valid HTML ensures your website renders correctly across all browsers and devices. When your HTML contains errors like unclosed tags, missing attributes, deprecated elements, or incorrect nesting, browsers have to guess what you meant, and different browsers guess differently. This leads to inconsistent layouts, accessibility problems, and potential SEO issues. The W3C HTML validator checks your markup against the official HTML specification and reports every error with an explanation of how to fix it.
Enter a URL or upload an HTML file using the tabs above. Select your encoding and doctype preferences, then click "Validate this page." The validator will analyze your markup and report any errors or warnings, along with the line numbers where they occur.
You do not need to achieve zero validation errors to have a functional website. Even major websites have validation warnings. Focus on fixing structural errors first: unclosed tags, missing alt attributes on images, and incorrect element nesting. These have the biggest impact on accessibility and cross-browser compatibility. Modern HTML5 is more forgiving than XHTML, but clean markup is always better for maintainability.
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