HTML validation checks whether your markup follows the HTML specification. It catches errors that browsers may attempt to recover from, but that still create fragile behavior.
Why HTML validation matters
Invalid markup can cause unpredictable layouts, inaccessible controls, broken forms, and parsing differences across browsers and assistive technologies.
How HTML validation works
Validators parse the document and report malformed structures, invalid attributes, duplicate IDs, and conformance issues. A site-wide workflow is more useful than checking isolated pages because templates and content combinations vary.
Code examples
<!-- Duplicate id values -->
<label for="email">Email</label>
<input id="email" type="email">
<input id="email" type="text"><!-- Fixed: unique ids -->
<label for="primary-email">Email</label>
<input id="primary-email" type="email">
<input id="backup-email" type="text">Related terms
HTML
Semantic HTML
Semantic HTML uses elements that describe meaning and structure, such as <main>, <nav>, and…
Accessibility
WCAG
WCAG (Web Content Accessibility Guidelines) is the global technical standard for making web content…
Accessibility
ARIA
ARIA (Accessible Rich Internet Applications) is a set of HTML attributes that add accessibility…
Help us improve this glossary term
Was this guide helpful?
Scan your site
Rocket Validator scans thousands of pages in seconds, detecting accessibility and HTML issues across your entire site.