Free DOCTYPE Checker

See if a standards mode HTML5 doctype is present.

Quick check for HTML5 doctype at the start of the document.

Try it now — free

Instant results for any public URL. We may email follow-up tips; no credit card required for this check.

Get a full-site SEO report in the dashboard →

About this free doctype checker

HTML5 doctype triggers standards mode in browsers. Missing or legacy doctypes cause quirks mode and inconsistent rendering.

Quickly see whether a page starts with a valid HTML5 doctype declaration.

What it checks

  • Presence of <!DOCTYPE html> at document start
  • Legacy doctype patterns if present

Why it matters for SEO

  • Consistent CSS and layout across browsers
  • Foundation for predictable DOM and performance debugging

How to use this tool

  1. Check main templates after theme migrations.
  2. Add HTML5 doctype if missing.
  3. Fix server-side includes that strip the first line.

Common issues we help you catch

  • DOCTYPE missing on partial HTML fragments served as pages
  • Uppercase or malformed doctype typos
  • XHTML legacy doctype on modern stacks

Related free SEO tools

Frequently asked questions

Does doctype affect SEO directly?
Rarely as a ranking factor, but quirks mode can break mobile layouts and UX — which hurts indirectly.