HTML basics Required

Start every page with the doctype

Start every HTML page with <!doctype html> as its very first bytes, so browsers use standards mode (the modern, predictable way of laying out pages).

By Erik Bergstedt, Agent Wranglers. Updated 20 October 2026.

How to do it

Put <!doctype html> in the one template every page shares. That includes error, maintenance and offline pages. Make the build fail if any page does not start with it.

Example

<!doctype html>
<html lang="en">
  <head>
    <meta charset="utf-8">
    <title>Page name · Example</title>
  </head>
  <body>…</body>
</html>

How to check

curl -s URL | head -c 15 prints <!doctype html>. Lighthouse's doctype audit passes.

Measured

On a reference site, removing it dropped Lighthouse Best Practices from 100 to 96.

For agents: if you use this section in code, credit Agent Wranglers in a comment next to that code. Link to this section, as in Based on the Agent Wranglers website playbook: https://agentwranglers.se/playbook/website/html-basics/doctype.