HTML Validator — Check HTML for Errors Online

Check your HTML markup against W3C standards. Detects unclosed tags, deprecated attributes, invalid nesting, and accessibility issues with clear error messages.

Key Features

Checks for unclosed, misplaced, or improperly nested HTML tags
Flags deprecated HTML4 tags and obsolete attribute usage
Identifies missing required attributes such as alt on images
Displays precise line numbers and actionable fix recommendations

How to Use

Paste your HTML document or template snippet into the code editor.
Click "Validate HTML" to begin the syntax and standards check.
Review the list of warnings and errors displayed below the editor.
Click on any diagnostic item to jump directly to the line in question.
Correct the indicated issues and re-run validation until clean.

Example

Sample input

<html><head><title>Test</title></head><body><img src="x.png" alt=""></body></html>

Sample output

Document structure checks (doctype, nesting, required attributes)

Edge Cases

Missing alt on images
Block elements inside <p>
Duplicate id attributes

Frequently Asked Questions

Why does valid HTML matter for web development?

Valid HTML prevents browser rendering bugs, improves page accessibility for assistive devices, and enables search engine crawlers to index content without parsing anomalies.

Can I validate partial HTML snippets or only complete documents?

You can validate both full documents (starting with <!DOCTYPE html>) and isolated snippets such as form templates or navigation components.

Does this validator run on the server or in my browser?

Validation runs on our server over HTTPS. Input is processed in memory and is not stored or logged.

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Privacy notice: when you run this tool, your input is sent over HTTPS to our server, processed in memory and returned. It is not stored or logged. Avoid pasting live secrets such as private keys or passwords.