HTML validation checks whether your website's code follows the official HTML specification. Valid code is more likely to display correctly across browsers, be properly indexed by search engines, and work with assistive technologies.
Google has stated that invalid HTML is not a direct ranking factor. However, invalid code often causes real problems that hurt SEO indirectly: broken layouts, missing content, inaccessible pages, and poor rendering.
Search engines parse the DOM (Document Object Model) that the browser creates from your HTML. Severely invalid code can produce an unexpected DOM that misses important content or structure.
Missing or duplicate IDs � Each page should have unique IDs. Duplicate IDs break JavaScript functionality and confuse CSS selectors.
Improperly nested tags � Tags must close in the reverse order they opened. Improper nesting causes browsers to render content unexpectedly.
Missing required attributes � Images without alt attributes fail accessibility checks. Form inputs without associated labels confuse screen readers.
Deprecated elements � Using outdated tags like center, font, or marquee indicates old code that should be modernised.
The W3C Markup Validation Service (validator.w3.org) checks your HTML against web standards. Browser developer tools also show validation warnings. Modern CMS platforms rarely produce invalid HTML, but custom code and third-party plugins often introduce errors.
Our free audit covers security, SEO, performance, mobile, design, and content.
Get your free audit