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ValidateHTML

Validate Your HTML Instantly

Paste your HTML code or enter a URL to check for errors, warnings, and best practices. Get a quality score with detailed reports.

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Ctrl+Enter to validate
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Instant Validation

Get results in milliseconds. No waiting, no page reloads. Just paste and validate.

Quality Score

Get a 0-100 score and letter grade for your HTML quality. Track your improvements over time.

URL Validation

Enter any URL and we'll fetch and validate the HTML for you. Check any live website.

How It Works

Paste or Enter URL

Paste your HTML code directly or enter a URL to validate.

Click Validate

Our engine checks against HTML standards and best practices.

Fix Issues

Review errors and warnings with line numbers and rule explanations.

What Does Our HTML Validator Check?

Unclosed & Mismatched Tags

Detects missing closing tags, improperly nested elements, and orphaned end tags that cause rendering issues.

Missing Required Attributes

Flags images without alt text, inputs without labels, and other elements missing attributes required by the HTML spec.

Deprecated Elements

Identifies obsolete HTML elements like <center>, <font>, and <marquee> that should be replaced with modern CSS.

Accessibility Issues

Catches WCAG violations including missing ARIA labels, empty headings, heading level skips, and missing form labels.

SEO-Critical Markup

Checks for missing DOCTYPE, lang attribute, viewport meta, canonical tags, and other markup that affects search rankings.

Attribute & Value Errors

Validates attribute values, detects duplicate IDs, invalid href formats, and incorrect boolean attributes.

Why Validate Your HTML?

Valid HTML ensures your website renders correctly across all browsers and devices. Invalid markup can cause layout issues, accessibility problems, and hurt your SEO rankings. Even small errors — like an unclosed <div> or a missing alt attribute — can cascade into major rendering problems.

Search engines like Google prefer well-structured HTML. Clean, valid code improves crawlability and helps search engines understand your content better, leading to better rankings. Google's own Lighthouse tool penalizes pages with invalid HTML in its accessibility and best practices audits.

ValidateHTML checks your code against modern HTML5 standards and best practices, catching common mistakes like missing alt attributes, unclosed tags, deprecated elements, incorrect nesting, and more. Unlike the W3C Validator, we give you a quality score and letter grade so you can track your improvements over time.

Whether you're building a personal site or a production web application, running your HTML through a validator before deployment catches issues early — when they're cheap to fix. Check out our guide to common HTML errors to learn about the most frequent issues developers face.

ValidateHTML vs W3C Validator

FeatureValidateHTMLW3C Validator
Quality Score (0-100)✓✗
Letter Grades (A-F)✓✗
Severity Filters✓✗
Modern UI✓✗
Instant Results✓Slower
URL Validation✓✓
Copy Results✓✗
Export PDF✓✗
Free, No Login✓✓

Both tools are free. ValidateHTML offers a more modern experience with scoring and grading features.

Frequently Asked Questions

What HTML errors does ValidateHTML check for?
ValidateHTML checks for a wide range of HTML issues including: unclosed tags, missing required attributes (like alt on images), deprecated HTML elements (like center, font), incorrect tag nesting, missing DOCTYPE declaration, invalid attribute values, accessibility issues, and more. We follow the latest HTML5 specification and web best practices.
Is ValidateHTML free to use?
Yes, ValidateHTML is completely free. You can validate unlimited HTML code snippets and URLs at no cost. No registration required.
How is the HTML quality score calculated?
Your score starts at 100 and is reduced based on unique issue types. Each unique error type deducts 15 points, each unique warning type deducts 5 points. Repeated occurrences of the same rule don't stack. Grades: A (90-100), B (75-89), C (50-74), D (25-49), F (below 25).
What is the difference between HTML errors and warnings?
Errors are violations of the HTML specification that can cause rendering problems, accessibility issues, or broken functionality. Warnings are best practice recommendations that won't break your page but could improve code quality, maintainability, and SEO.
Does HTML validation affect SEO?
Yes. While Google doesn't directly penalize invalid HTML, clean markup improves crawlability, reduces rendering errors, and ensures search engines can properly parse your content. Valid HTML also improves accessibility, which is increasingly important for SEO.
Can I validate HTML from a live website?
Yes. Switch to URL mode, enter any website address (you don't need to type https://), and we'll fetch the page HTML and validate it for you. This works with any publicly accessible URL.
What is a good HTML quality score?
A score of 90-100 (Grade A) means your HTML is clean and follows best practices. 75-89 (Grade B) is good with minor issues. Below 50 (Grade D or F) indicates significant problems that should be fixed. Most production websites score between 60-85.
How is ValidateHTML different from the W3C Validator?
ValidateHTML offers a modern interface with a 0-100 quality score, letter grades, severity filters, copy/export features, and instant results. The W3C Validator is the official standard checker but has a dated interface and no scoring system. Both are free and both catch HTML errors.