W3C Markup Validation Service

The W3C Markup Validation Service is the official tool from the World Wide Web Consortium for checking the conformance of HTML and XHTML documents against web standards. Users can validate pages by entering a URL (URI), uploading a file, or pasting markup directly. The validator parses the document and reports errors and warnings with line numbers, suggesting fixes for each issue. It checks document type declaration, character encoding, element nesting rules, attribute quoting, required attributes, and DOCTYPE conformance. The service also offers an API for automated integration into build pipelines and CI/CD workflows. Maintaining valid markup improves cross-browser compatibility, accessibility for screen readers, search engine crawlability, and reduces the likelihood of rendering quirks. For professional web developers and quality assurance engineers, running pages through W3C validation is a standard best practice.