HTML Checker (Tags, Accessibility, SEO Basics)
Check HTML for unclosed or mismatched tags, duplicate IDs, images without alt text, missing lang/title/viewport and other common mistakes.
How to use
- 1Paste your HTML (a full page or a fragment).
- 2Fix the issues listed — each shows the line number.
How it's calculated
Tags are matched with a stack (void elements like <img> and <br> need no closing tag). Page-level checks run only when the HTML includes <html>.
Frequently asked questions
Is this the same as the W3C validator?
No — it catches the most common real-world mistakes quickly. For full spec conformance, also run validator.w3.org.
Why does alt text matter?
Screen readers read it aloud and search engines use it. Decorative images should have an empty alt="".
Are self-closing tags like <br/> OK?
Yes — HTML allows the trailing slash on void elements; it's simply ignored.