HTML Validator

Validate HTML online using W3C standards. Check any URL or paste HTML code to find errors, warnings, and accessibility issues with line numbers.

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APIPOST /api/v1/web/validate
4.5(11 votes)
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Key Features

100% Free

No registration required, unlimited checks

Instant Results

Real-time analysis with detailed output

REST API Access

Integrate into your workflow via API

Accurate Data

Live queries to authoritative sources

What is HTML Validator?

The HTML validator checks web pages against W3C (World Wide Web Consortium) standards, the official specification for HTML markup. Enter a URL to validate a live page or paste HTML code directly for analysis. The tool identifies markup errors (unclosed tags, missing required attributes, improperly nested elements), warnings about deprecated elements and non-standard usage, accessibility issues (missing alt attributes, improper heading hierarchy, missing form labels), and best practice violations.

Each issue includes the line number, code excerpt, and explanation of what's wrong and how to fix it. Valid HTML is important because it ensures consistent rendering across all browsers and devices, helps search engine crawlers parse and index your content correctly, enables screen readers and assistive technologies to interpret your pages for accessibility, reduces unexpected layout shifts and rendering bugs, and contributes to better Core Web Vitals scores. This free online HTML checker is used by web developers debugging markup issues, QA teams validating pages before deployment, accessibility specialists auditing pages for WCAG compliance, and SEO specialists ensuring crawlable, well-structured pages.

How to Use

  1. 1Choose validation mode: URL (enter a web address to validate a live page) or HTML (paste code directly)
  2. 2Enter the URL or paste your HTML markup into the input area
  3. 3Click 'Validate' to run the check against W3C HTML standards
  4. 4Review errors (must fix — invalid markup), warnings (should fix — best practice issues), and info messages
  5. 5Click on any issue to see the exact line number, code excerpt, and explanation of the problem
  6. 6Fix the issues in your source code and re-validate to confirm they're resolved

Who Uses This

System Administrators

Monitor and troubleshoot infrastructure

Developers

Debug network issues and integrate via API

SEO Specialists

Verify domain configuration and performance

Security Analysts

Audit and assess network security

Frequently Asked Questions

Why should I validate my HTML markup?
Valid HTML ensures your pages work correctly across all browsers (Chrome, Firefox, Safari, Edge), render predictably on all devices (desktop, mobile, tablet), are accessible to screen readers and assistive technologies, are properly parseable by search engine crawlers for SEO, and don't suffer from unexpected layout shifts or rendering bugs. While browsers are forgiving and will try to render broken HTML, the results are unpredictable and inconsistent. One browser might handle a missing closing tag gracefully while another breaks the layout entirely.
What is W3C validation and which standards does it check?
W3C (World Wide Web Consortium) is the international organization that develops web standards, including the HTML specification. W3C validation checks your markup against the official HTML5 specification (or HTML Living Standard maintained by WHATWG). It verifies proper element nesting, required attributes (like alt on images), valid attribute values, correct document structure (DOCTYPE, head, body), and proper use of semantic elements. The validation follows the same rules that browsers use to parse HTML.
Does HTML validation directly affect SEO rankings?
Google has stated they don't require perfect HTML validation for ranking. However, valid markup indirectly benefits SEO in several ways: it helps Googlebot correctly parse and index your page content (broken HTML can hide content from crawlers), it reduces layout shifts that hurt Core Web Vitals (CLS score), it ensures structured data and meta tags are properly rendered, and it improves page load performance by avoiding browser error recovery overhead. While you don't need a zero-error validation score, fixing critical markup errors that affect content parsing and rendering is worth the effort.
What are the most common HTML validation errors?
The most frequently found errors include: unclosed elements (missing closing tags like a forgotten closing div or p tag), missing required attributes (img without alt, links without href), improperly nested elements (block elements inside inline elements), duplicate ID attributes (IDs must be unique within a page), deprecated elements and attributes (using obsolete HTML4 tags like center or font), missing DOCTYPE declaration, and invalid attribute values (like non-numeric values where numbers are expected). Most of these are easy to fix once identified.
Can I validate HTML from a local file or development server?
Use the HTML paste mode to validate code from local files — copy your HTML source and paste it directly into the validator. For live pages on a development server, the URL mode works if the server is publicly accessible. If your dev server is behind a firewall or on localhost, copy the rendered HTML source (View Page Source in your browser) and paste it into HTML mode. This tool processes everything in real-time and doesn't store your code.

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API Endpoint

POST /api/v1/web/validate
View Documentation