Require A Doctype Declaration

html.doctype_declaration

Severity

low

Remediation Complexity

low

Remediation Risk

low

Remediation Effort

low

Resource

Reliability

Language

Html

Description

Flags a document that declares no <!DOCTYPE> at all.

Rationale

The doctype is the first thing a browser reads. Without it the browser drops into quirks mode, where legacy box-model and layout bugs are emulated for backwards compatibility, so a page that renders correctly in standards mode can shift or break. A missing doctype therefore makes layout non-deterministic across browsers.

<html lang="en">      <!-- FLAW — document has no doctype declaration -->
  <head><title>Hi</title></head>
  <body></body>
</html>

<!DOCTYPE html>       <!-- OK — doctype declared -->
<html lang="en">
  <head><title>Hi</title></head>
  <body></body>
</html>

Remediation

Begin the document with a doctype declaration; the modern <!DOCTYPE html> selects standards mode in every browser.

References

MDN — Doctype