HTML Security Scanner
Our HTML security scanner helps you scan HTML code for security vulnerabilities. Check for XSS attacks, missing CSP headers, insecure resources, and OWASP Top 10 vulnerabilities. Test for inline event handlers, dangerous JavaScript patterns, external links without rel="noopener", and sensitive data exposure. All scanning happens locally in your browser.
- OWASP
- Top 10 2021
- Standards
- OWASP
- Mode
- In-browser
- Price
- Free
OWASP Top 10
Tests against OWASP Top 10 2021 standards for complete security compliance.
XSS & CSP
Detects XSS vulnerabilities and validates Content Security Policy implementation.
100% Private
Everything runs locally. Your HTML never leaves your device.
Scan HTML security online
Paste your HTML code, click Scan Security, and review the OWASP Top 10 vulnerability results with errors, warnings, and suggestions.
Demo fetch uses a CORS-friendly approach only if the target allows it.
Privacy-first
This page processes content locally in your browser (no upload).
What is HTML Security Scanning?
HTML security scanning is the process of testing HTML code for security vulnerabilities and potential attack vectors. An HTML security scanner analyzes your code to detect XSS (Cross-Site Scripting) vulnerabilities, missing security headers, insecure resources, and other OWASP Top 10 security issues that could expose your website and users to attacks.
When you build websites, it's essential to ensure they're secure against common web vulnerabilities. Inline event handlers (onclick, onerror) can lead to XSS attacks. Missing Content Security Policy (CSP) headers leave your site vulnerable to injection attacks. External links without rel="noopener" can enable tabnabbing attacks. An HTML security scanner helps you identify and fix these issues to achieve OWASP Top 10 compliance and protect your website from security threats.
✗Invalid HTML
<!DOCTYPE html>
<html>
<head>
<title>Example</title>
</head>
<body>
<h1>Welcome</h1>
<p>This paragraph is not closed
<img src="image.jpg">
<a href="link.html">Click here
</body>
</html>Missing closing tags, missing alt text, unclosed elements
✓Valid HTML
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Example</title>
</head>
<body>
<h1>Welcome</h1>
<p>This paragraph is closed.</p>
<img src="image.jpg" alt="Description">
<a href="link.html">Click here</a>
</body>
</html>All tags closed, proper structure, accessibility attributes
What Does HTML Validation Check?
- •Syntax Errors: Missing closing tags, mismatched tags, invalid characters, and malformed HTML structure
- •Required Elements: DOCTYPE declaration, html tag, head tag, title tag, body tag, and other essential HTML5 elements
- •Accessibility Issues: Missing alt text on images, missing lang attribute, improper heading hierarchy, missing ARIA labels, and other WCAG compliance issues
- •SEO Problems: Missing meta description, missing Open Graph tags, improper heading structure, missing title tag, and other SEO best practices
- •Performance Warnings: Missing lazy loading on images, oversized images, missing optimization attributes, and other performance issues
- •Security Issues: External links missing rel="noopener", insecure resources, and other security best practices
According to OWASP Top 10 2021, XSS attacks are among the most common web vulnerabilities. Inline event handlers, dangerous JavaScript patterns, and missing CSP headers can lead to serious security breaches. Our HTML security scanner tests for OWASP Top 10 compliance to ensure your website is protected against common attacks.
Modern web development workflows should include HTML security scanning as a standard step. Whether you're building a new website, maintaining existing code, or learning HTML, using an HTML security scanner helps ensure your code is secure, protected against attacks, and compliant with security best practices. For more information on security standards, see the OWASP Top 10 2021, MDN Web Security documentation, and Google's web security guide.
HTML Security Impact
Real data showing the importance of scanning HTML for security vulnerabilities
Validation Statistics
According to OWASP research, over 75% of websites have security vulnerabilities that could be exploited by attackers. XSS attacks, missing CSP headers, insecure resources, and external links without proper security attributes are common problems. Regular HTML security scanning helps catch and fix these issues to achieve OWASP Top 10 compliance and protect your website from attacks.
Why Scan HTML Security?
Scanning HTML for security vulnerabilities is essential for building secure, protected, and trustworthy websites. Here's why you should make security scanning part of your development workflow:
Ensure Browser Compatibility
Invalid HTML can render differently across browsers. Chrome, Firefox, Safari, and Edge may handle errors inconsistently, leading to layout breaks, missing content, or broken functionality. Valid HTML ensures consistent rendering across all browsers and devices, reducing cross-browser testing time and user complaints.
Improve Accessibility
Insecure HTML exposes websites to XSS attacks and other vulnerabilities. Inline event handlers allow attackers to inject malicious scripts. Missing CSP headers leave sites vulnerable to injection attacks. External links without rel="noopener" enable tabnabbing attacks. Secure HTML with proper CSP implementation, safe JavaScript practices, and secure resource loading is the foundation of OWASP Top 10 compliance. This is not just best practice—it's essential for protecting your users and your business from security breaches.
Boost SEO Rankings
Search engines like Google prefer valid, well-structured HTML. Missing meta tags, improper heading hierarchy, and invalid structure can hurt your search rankings. Valid HTML with proper semantic structure helps search engines understand and index your content better, potentially improving your rankings and organic traffic.
Catch Errors Early
HTML validation catches errors before they cause problems in production. Missing closing tags, invalid attributes, and structural errors can lead to broken layouts, JavaScript failures, and user experience issues. Validating during development saves debugging time and prevents costly fixes after deployment.
Improve Performance
Invalid HTML can cause browsers to spend extra time parsing and fixing errors, slowing down page rendering. Valid HTML renders faster, improving Core Web Vitals metrics like First Contentful Paint (FCP) and Largest Contentful Paint (LCP). Faster pages provide better user experience and can improve search rankings.
Enhance Security
Valid HTML helps prevent security vulnerabilities. Missing rel="noopener" on external links can expose your site to tabnabbing attacks. Invalid HTML can also make your site more vulnerable to XSS attacks. Validating HTML helps ensure you're following security best practices and protecting your users.
How It Works
Our HTML validator uses client-side parsing and rule checking to validate your HTML code. Here's how the validation process works:
Parse HTML Structure
The validator parses your HTML code to identify all tags, attributes, and structure. It builds a tree representation of your document and checks for proper nesting and hierarchy.
Check Syntax Errors
The validator checks for missing closing tags, mismatched tags, invalid attributes, missing required elements (DOCTYPE, html, head, body, title), and other syntax errors that break HTML validity.
Validate Accessibility
The validator checks for accessibility issues including missing alt text on images, missing lang attribute, improper heading hierarchy (h1 should be first, no skipped levels), missing ARIA labels, and other WCAG compliance issues.
Check SEO & Performance
The validator checks for SEO issues (missing meta description, missing Open Graph tags, improper heading structure) and performance warnings (missing lazy loading, security issues with external links). It generates a comprehensive report with errors, warnings, and suggestions.
Best Practices for HTML Security
Follow these best practices to ensure your HTML code is secure and protected against vulnerabilities:
Always Include DOCTYPE
Every HTML document should start with <!DOCTYPE html>. This tells browsers which HTML version to use and ensures proper rendering. Without it, browsers may enter quirks mode, causing inconsistent rendering.
✅ DO: <!DOCTYPE html>
❌ DON'T: Skip DOCTYPE declaration
Close All Tags Properly
Every opening tag must have a corresponding closing tag (except self-closing tags like <img>, <br>). Mismatched or unclosed tags can break layout and functionality.
Test regularly: Validate HTML after major changes, before deployment, and as part of your build process
Use Semantic HTML
Use semantic HTML5 elements like <header>, <nav>, <main>, <section>, <article>, and <footer>. These improve accessibility, SEO, and code maintainability.
Semantic benefits: Better accessibility • Improved SEO • Easier maintenance • Clearer code structure
Add Accessibility Attributes
Always implement Content Security Policy (CSP) headers, avoid inline event handlers, use HTTPS for all resources, and add rel="noopener" to external links. These are required for OWASP Top 10 compliance.
Accessibility checklist: Alt text on images • Lang attribute • Proper headings • ARIA labels • Keyboard navigation
Include Essential Meta Tags
Add essential meta tags for SEO and functionality: charset, viewport, description, and Open Graph tags for social sharing. These improve SEO rankings and user experience.
Essential meta tags: charset="UTF-8" • viewport for mobile • description for SEO • og:tags for social
Validate Regularly
Validate your HTML code regularly—after major changes, before deployment, and as part of your build process. Use automated validation in CI/CD pipelines to catch errors early. Regular validation prevents issues from accumulating and becoming harder to fix.
Validation schedule: After code changes • Before deployment • In CI/CD pipeline • During code reviews
Frequently Asked Questions
How do I validate HTML code?
Paste your HTML code into the validator, click Validate, and review the results. The tool checks for syntax errors, missing tags, accessibility issues, SEO problems, and performance warnings. All validation happens locally in your browser for complete privacy.
What HTML errors does the validator detect?
Our HTML validator detects missing DOCTYPE, unclosed tags, mismatched closing tags, missing required elements (html, head, body, title), invalid attributes, and structural issues. It also checks for accessibility problems like missing alt text and SEO issues like missing meta tags.
Do you store my HTML code?
No. This HTML validator processes everything locally in your browser. Your code never leaves your device, ensuring complete privacy and security. No server uploads, no data storage, no privacy concerns.
What's the difference between errors and warnings?
Errors are critical issues that break HTML validity or functionality (missing closing tags, invalid structure). Warnings are important but non-critical issues (missing alt text, missing meta tags). Suggestions are best practices for better SEO, accessibility, and performance.
Does this validator check for accessibility issues?
Yes. Our HTML validator checks for accessibility issues including missing alt text on images, missing lang attribute, improper heading hierarchy, missing ARIA labels, and other WCAG compliance issues. This helps ensure your HTML is accessible to all users.
Can I validate HTML from a URL?
Yes. You can fetch HTML from a URL using the fetch feature, though it may be blocked by CORS policies. Alternatively, copy the HTML source code from your browser's developer tools and paste it into the validator for complete validation.
What SEO issues does the validator check?
The validator checks for missing meta description, missing Open Graph tags, improper heading hierarchy (h1 should be first, no skipped levels), missing title tag, and other SEO best practices. These checks help improve your search engine rankings.
Is this validator based on W3C standards?
Yes. Our HTML validator follows W3C HTML5 standards and checks for compliance with official HTML specifications. It validates syntax, structure, and best practices according to W3C guidelines and modern web standards.
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