Create account
SEO and auditingChecked through WebDiag API

HTML Markup Validator

Enter a URL to get a practical HTML structure check. This is not a fake W3C clone, but deterministic diagnostics for common template issues.

  • HTML structure
  • Duplicate IDs
  • Mobile viewport
Online tool

Try the tool

Enter a URL: WebDiag will run a safe network check and show the result.

Page URL

HTML markup

After the check, doctype, lang, viewport, duplicate IDs, unmatched/unclosed tags, and recommendations will appear here.

Network check through backend

For HTTP/SEO tools, the URL is sent to the WebDiag API, where SSRF protection, DNS/IP policy, and redirect limits are applied.

Reviewed: 2026-07-17
01

How to use it

  1. 1

    Paste the page URL.

  2. 2

    Run the HTML markup check through the backend.

  3. 3

    Fix doctype, lang, title, viewport, duplicate IDs, and unmatched tag issues.

02

What the tool can do

  • Checks doctype, html/head/body, html lang, title, and viewport.
  • Finds duplicate id attributes, unexpected closing tags, and unclosed non-void tags.
  • Uses safe backend HTML fetching with SSRF protection.
03

Common use cases

  • Check a template after a CMS migration or SSR/SSG release.
  • Find duplicate IDs that break anchors, labels, ARIA, and scripts.
  • Quickly see basic HTML issues before an SEO audit.
04

How it works inside

The parser analyzes the raw HTML response and does not execute JavaScript.

The report is intentionally framed as practical inspection so it does not imitate an external W3C validator.

05

Questions and answers

Does this replace the W3C validator?

No. This is a built-in WebDiag check for basic structure and common errors. Full conformance validation requires a separate integration or specialized validator.

Why do duplicate IDs matter?

They break fragment links, label/for, ARIA references, and JavaScript selectors, so this is a useful HTML quality signal.