HTML Performance Analyzer
Our HTML performance analyzer helps you analyze HTML code for performance issues. Check for missing lazy loading, improper image optimization, render-blocking scripts, missing resource hints, and other performance best practices. Test for Core Web Vitals optimization, image loading attributes, script loading, and resource hints. All analysis happens locally in your browser.
- Checks
- 10+ Performance
- Standards
- Core Web Vitals
- Mode
- In-browser
- Price
- Free
Core Web Vitals
Validates LCP, FID, and CLS optimizations for optimal Core Web Vitals scores.
Image Optimization
Checks lazy loading, width/height attributes, and decoding attributes.
100% Private
Everything runs locally. Your HTML never leaves your device.
Analyze HTML performance online
Paste your HTML code, click Analyze Performance, and review the performance analysis 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 Performance Analysis?
HTML performance analysis is the process of testing HTML code for performance issues and best practices. An HTML performance analyzer analyzes your code to detect missing lazy loading, improper image optimization, render-blocking scripts, missing resource hints, and other performance issues that could impact your Core Web Vitals scores and website loading speed.
When you build websites, it's essential to ensure they're optimized for performance. Missing lazy loading on images increases Largest Contentful Paint (LCP). Render-blocking scripts delay First Input Delay (FID). Missing width/height attributes on images cause Cumulative Layout Shift (CLS). An HTML performance analyzer helps you identify and fix these issues to achieve better Core Web Vitals scores and improve your website's loading speed.
✗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 lazy loading, no width/height attributes, render-blocking scripts
✓Optimized HTML
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Example</title>
</head>
<body>
<h1>Welcome</h1>
<p>This paragraph is closed.</p>
<img src="image.jpg" alt="Description" loading="lazy" width="800" height="600" decoding="async">
<script src="script.js" async></script>
</body>
</html>Lazy loading, width/height attributes, async scripts, performance optimized
What Does HTML Performance Analysis Check?
- •Core Web Vitals (LCP): Missing lazy loading on images, oversized images, missing image optimization attributes
- •Core Web Vitals (CLS): Missing width/height attributes on images, missing image dimensions, layout shift causes
- •Core Web Vitals (FID): Render-blocking scripts, missing async/defer attributes, blocking JavaScript
- •Image Optimization: Missing lazy loading, missing width/height, missing decoding attribute, image format optimization
- •Resource Hints: Missing preconnect, missing dns-prefetch, missing preload for critical resources
- •Script Loading: Render-blocking scripts, missing async/defer, third-party script optimization
According to Google Core Web Vitals, proper image optimization and script loading are essential for fast loading times. Missing lazy loading, render-blocking scripts, and missing resource hints can significantly impact your Core Web Vitals scores. Our HTML performance analyzer tests for performance best practices to ensure your website is optimized for Core Web Vitals.
Modern web development workflows should include HTML performance analysis as a standard step. Whether you're building a new website, maintaining existing code, or learning HTML, using an HTML performance analyzer helps ensure your code is optimized, fast-loading, and compliant with Core Web Vitals best practices. For more information on performance standards, see the Google Core Web Vitals guide, MDN Web Performance documentation, and Google's Web Performance guide.
HTML Performance Impact
Real data showing the importance of analyzing HTML for performance optimization
Validation Statistics
According to Google Web Vitals research, over 70% of websites have performance issues that could be improved. Missing lazy loading on images, render-blocking scripts, missing resource hints, and improper image optimization are common problems. Regular HTML performance analysis helps catch and fix these issues to achieve better Core Web Vitals scores and improve your website's loading speed.
Why Analyze HTML Performance?
Analyzing HTML for performance issues is essential for building fast, optimized, and user-friendly websites. Here's why you should make performance analysis 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
Unoptimized HTML prevents websites from loading quickly. Missing lazy loading on images increases Largest Contentful Paint (LCP). Render-blocking scripts delay First Input Delay (FID). Missing width/height attributes on images cause Cumulative Layout Shift (CLS). Performance-optimized HTML with proper image optimization, script loading, resource hints, and HTML optimization is the foundation of Google Core Web Vitals. This is not just best practice—it's essential for improving your website's loading speed and user experience.
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 Performance
Follow these best practices to ensure your HTML code is optimized for performance:
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 include lazy loading on images (loading="lazy"), width/height attributes on images (prevents CLS), async/defer on scripts (prevents render-blocking), resource hints (preconnect, dns-prefetch), and viewport meta tag (mobile optimization). These are required for Google Core Web Vitals.
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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