</> Web Validators

HTML Validator

Check HTML for markup errors, accessibility issues and common technical SEO problems.

Based on: WHATWG HTML Living Standard

About this validation

Checks HTML markup against the current HTML Living Standard: invalid elements, malformed attributes, and structural errors.

Protocol / standard:
Validation engine:
html-validate
Rules reviewed:
Aug 31, 2026

Standards and vendor-documented behavior can change. This reflects the source as last reviewed above, not a permanent guarantee.

Ctrl/Cmd + Enter

What the HTML Validator Checks

HTML Structure

  • DOCTYPE, head, body
  • Element nesting
  • Unclosed / mismatched tags

Attributes

  • Duplicate IDs
  • Invalid attribute values
  • Broken attribute syntax

Accessibility

  • Missing alt text
  • Form fields without labels
  • Empty links and buttons

Metadata

  • Charset & viewport
  • Title & lang attribute
  • Document language

SEO Markup

  • Canonical tags
  • Heading structure
  • Robots directives

Deprecated HTML

  • Legacy elements
  • Legacy attributes
  • Non-standard markup

How It Works

  1. 1

    Paste HTML or enter a URL

  2. 2

    Run validation

  3. 3

    Review and fix issues

Common HTML Errors

Missing alt

Wrong

<img src="photo.jpg">

Correct

<img src="photo.jpg" alt="Mountain landscape">

Invalid nesting

Wrong

<p><div>Content</div></p>

Correct

<div><p>Content</p></div>

FAQ

What is HTML validation?

HTML validation checks a document's markup against the HTML specification and common best practices, flagging structural errors, invalid attributes, and accessibility problems.

Can I validate a live website?

Yes. Switch to the "Enter URL" tab, provide a public page address, and the page is fetched server-side and checked automatically.

Does valid HTML improve SEO?

Clean, well-structured markup makes it easier for search engines to parse a page's content and metadata, but validity alone is one of many ranking factors.

What is the difference between an error and a warning?

Errors are issues that break the HTML specification or cause real problems for users and assistive technology. Warnings are best-practice recommendations that are worth fixing but won't break the page.

Is the validator free?

Yes, every tool on this site is free to use without an account.