HTML Validator - Validate HTML Online Free
Validate your HTML code online for errors, warnings, and best practice issues. Free client-side HTML validator, no data sent to server.
Input
Validation Results
What Is an HTML Validator?
An HTML validator checks your HTML markup for common errors, warnings, and best-practice violations. It detects problems like unclosed tags, missing attributes, deprecated elements, and missing DOCTYPE declarations. Catching these issues early prevents rendering bugs and accessibility problems.
This tool runs entirely in your browser — no data is sent to a server. It checks for structural errors, accessibility warnings (e.g. missing alt attributes), and deprecated HTML elements.
How to Use This Tool
Paste or Upload HTML
Paste your HTML into the left editor, or click Upload to load a file. Click Sample to try an example.
Review Results
The right panel shows validation results automatically. Errors are prefixed with ✗ ERROR, warnings with ⚠ WARNING, and informational messages with ℹ INFO.
Fix Issues and Re-validate
Correct the issues in the left editor and the results update in real-time. To format your HTML for readability, try the HTML Formatter tool.
HTML Validation Example
Below is a snippet with common issues and the resulting validation output:
HTML with issues
Validation output
Why HTML Validation Matters
Invalid HTML can cause browsers to enter quirks mode, leading to inconsistent rendering across browsers. Missing alt attributes on images cause accessibility failures. Unclosed tags can break layout in unexpected ways.
For authoritative validation, use the W3C Markup Validation Service. This tool provides quick client-side checks for common issues.
Frequently Asked Questions
Does this replace the W3C Validator?
No. This tool provides quick client-side checks for common issues. For authoritative validation against the HTML specification, use the W3C Markup Validation Service.
Is my HTML sent to a server?
No. All validation runs entirely in your browser. No data is sent anywhere.
What kinds of errors does it detect?
It detects unclosed tags, closing tags without matching opening tags, missing alt attributes on images, deprecated elements (font, center, marquee, etc.), and missing DOCTYPE declarations.
Related Tools
See the W3C Markup Validation Service for authoritative HTML validation. The HTML Living Standard is the reference specification.