HTML Validator
Validate HTML structure online. Check for missing required attributes, unclosed tags, deprecated elements, accessibility issues, and nesting errors. Free, no server upload.
What is HTML Validator?
HTML Validator checks your HTML markup for structural errors, missing attributes, deprecated elements, and accessibility problems entirely in the browser. It scans for issues like missing alt attributes on images, unclosed tags, invalid nesting, missing required attributes, and use of obsolete HTML elements — helping you ship cleaner, more accessible web pages.
How to Use HTML Validator
- Paste your HTML snippet or full page into the input area
- Click 'Validate' to run all checks
- Review the list of errors, warnings, and informational messages
- Fix each issue and re-validate until no issues remain
HTML Validator Features
- Detects missing alt attributes on images (accessibility)
- Checks for unclosed or improperly nested tags
- Flags deprecated HTML elements (font, center, marquee, etc.)
- Validates required attributes (src on img, href on a, etc.)
- Checks document structure (missing lang, title, meta charset)
Frequently Asked Questions
Does this follow the official W3C spec?
This tool applies a practical subset of HTML validation rules covering the most common and impactful issues. For full W3C spec conformance testing, use validator.w3.org.
Can I validate a full HTML page?
Yes. Paste the complete HTML document including the <!DOCTYPE html> declaration for the most accurate results.
What types of issues does it catch?
Errors (broken structure), warnings (deprecated or risky usage), and info-level suggestions (accessibility improvements and best practices).