HTML Validator (Basic)
HTML validator online. Check for unclosed tags, invalid nesting, duplicate IDs, and common HTML errors. Validate HTML free in your browser.
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Basic Validation
This validator checks for common issues like unclosed tags, duplicate IDs, and nesting problems. It supports HTML5, SVG, and MathML elements. Not W3C-compliant.
Features
- Detect unclosed HTML tags
- Check for invalid tag nesting
- Find duplicate ID attributes
- Identify self-closing tag errors
- Validate attribute syntax
- Real-time validation with error highlighting
Common Use Cases
- Debug HTML rendering issues in browsers
- Validate HTML before deploying to production
- Check email template HTML for compatibility
- Find structural errors in scraped or generated HTML
- Ensure accessibility compliance with proper structure
HTML Validation and Error Detection
HTML validation is the process of checking HTML code against standard rules to ensure it's well-formed and error-free. Valid HTML renders consistently across browsers and is essential for accessibility and SEO.
Common HTML errors include:
- Unclosed tags: Opening tags like
<div>without matching</div> - Invalid nesting: Block elements inside inline elements (e.g.,
<a><div></div></a>) - Duplicate IDs: Multiple elements with the same
idattribute - Self-closing errors: Non-void elements self-closed like
<div /> - Missing required attributes:
<img>withoutaltorsrc
Why validation matters:
- Browser compatibility: Valid HTML renders consistently across all browsers
- Accessibility: Screen readers rely on proper HTML structure
- SEO: Search engines prefer well-formed HTML
- Debugging: Validation helps identify the root cause of rendering issues
Note: This validator performs basic structural checks. For full HTML5 specification compliance, use the W3C Markup Validation Service.
Examples
Valid - Valid HTML
<div id="container"><p>Hello World</p></div> Invalid - Unclosed Tag (Invalid)
<div id="container"><p>Hello World</div> Invalid - Duplicate ID (Invalid)
<div id="box">A</div><div id="box">B</div>Frequently Asked Questions
Why does my HTML work in browsers but shows errors here?
Modern browsers are very forgiving and auto-correct many HTML errors. However, relying on browser error correction can lead to unexpected behavior, accessibility issues, and SEO problems.
What's the difference between this and W3C validator?
This tool performs basic structural validation (unclosed tags, nesting, duplicate IDs). The W3C validator checks full HTML5 specification compliance including semantic elements, attributes, and ARIA roles.
Can invalid HTML affect SEO?
Yes, search engines prefer well-formed HTML. While they can parse invalid HTML, errors may prevent them from understanding your content structure, affecting rankings.
Should I validate HTML in email templates?
Absolutely! Email clients are less forgiving than browsers. Invalid HTML can break layouts in Outlook, Gmail, or mobile email apps. Always validate email template HTML.
Are self-closing tags like <div /> valid?
No, in HTML5 only void elements (img, br, hr, input, etc.) can be self-closing. Non-void elements like div, span, or p must have explicit closing tags.
💡 Tips
- Validate HTML early in development to catch errors before they compound
- Use proper semantic HTML5 elements (header, nav, article, section) for better accessibility and SEO
- Always include alt attributes on images for accessibility—validators will remind you
- Keep IDs unique and use classes for styling multiple elements with the same style