Rules / html rules

html.doctype

Default

error

Plugin

html

A page starts with <!doctype html>.

Why: Without the doctype, browsers render the page in quirks mode, which changes the layout and can hide content.

When off: pages without a doctype pass.

<!-- ✗ Fails -->
<html lang="en">
  ...
</html>

<!-- ✓ Passes -->
<!doctype html>
<html lang="en">
  ...
</html>

References

• HTML spec: The DOCTYPE: A document must start with a DOCTYPE.
• MDN: Quirks mode and standards mode: Without <!doctype html>, browsers render the page in quirks mode.