HTML check: Obsolete doctype. Expected “<!DOCTYPE html>”.

About This HTML Issue

The doctype declaration tells the browser which version of HTML the document is written in and determines how the browser renders the page. Older HTML versions required long, complex doctype strings that referenced Document Type Definitions (DTDs). With the introduction of HTML5, the doctype was simplified to just <!DOCTYPE html>, which is now the only recommended doctype for modern web pages.

When a browser encounters an obsolete doctype—or no doctype at all—it may switch into “quirks mode,” a legacy rendering mode that emulates old browser behavior. In quirks mode, CSS box models, layout calculations, and other rendering behaviors can differ significantly from standards mode, leading to inconsistent appearance across browsers. Using the modern <!DOCTYPE html> ensures the browser operates in “standards mode,” giving you predictable and consistent rendering.

Beyond rendering concerns, using an obsolete doctype means your document is referencing outdated specifications. Modern HTML validators, including the W3C Nu HTML Checker, are designed to validate against the current HTML Living Standard. Legacy doctypes may cause the validator to misinterpret your markup or report errors that don’t apply, making it harder to maintain clean, valid code.

To fix this, simply replace whatever doctype declaration appears at the top of your HTML file with <!DOCTYPE html>. This single change is backward-compatible—it works in all modern browsers and even in Internet Explorer 6 and later. No DTD reference or version number is needed.

Examples

Obsolete doctypes that trigger this issue

These are common legacy doctypes that the validator will flag:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
  "http://www.w3.org/TR/html4/loose.dtd">
<html>
  <head>
    <title>My Page</title>
  </head>
  <body>
    <p>Hello world</p>
  </body>
</html>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
  "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
  <head>
    <title>My Page</title>
  </head>
  <body>
    <p>Hello world</p>
  </body>
</html>

Correct HTML5 doctype

Replace the obsolete doctype with the simple, modern declaration:

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="utf-8">
    <title>My Page</title>
  </head>
  <body>
    <p>Hello world</p>
  </body>
</html>

Key points about the modern doctype

It must appear on the very first line of the document, before the <html> tag. No comments or whitespace should precede it.
The declaration is case-insensitive—<!DOCTYPE html>, <!doctype html>, and <!Doctype Html> are all valid, though lowercase or uppercase conventions are most common.
Unlike older doctypes, it does not reference a DTD URL or version number.
Adding a lang attribute to the <html> element and a <meta charset="utf-8"> tag in the <head> are best practices that pair well with the modern doctype for a fully standards-compliant document.

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Learn more:

WHATWG Spec

HTML Living Standard: The DOCTYPE

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