DOCTYPE Declaration

The DOCTYPE declaration is a line at the very top of an HTML document that tells the browser which rendering mode to use. In modern HTML it is simply <!DOCTYPE html>. It is not an HTML element and is case-insensitive.

Why it matters

Without a valid doctype, browsers fall back to quirks mode, which imitates decades-old behavior. In quirks mode the box model, table sizing, line heights and some layout details differ from the standard, so pages look and behave unpredictably. Older HTML 4 and XHTML doctypes were long strings referencing a document type definition; HTML5 reduced it to a short form that triggers standards mode in every current browser.

Example

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>Example</title>
</head>
<body>Hello</body>
</html>

You can check the mode in the console with document.compatMode, which returns "CSS1Compat" for standards mode and "BackCompat" for quirks mode.

Common mistakes

• Putting a comment or whitespace-generating code before the doctype, which can trigger quirks mode in some browsers.
• Leaving it out of templates or generated pages.
• Copying an old XHTML doctype from legacy code.
• Debugging odd layouts without checking the rendering mode.

Related

• HTML
• HTML5
• Box model
• Cross-browser compatibility

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