Standard (and valid) DOCTYPE definitions (DTDs)

Last updated: 2006/03/17

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Web pages should include a valid DOCTYPE definition (DTD) at the beginning of the HTML document, not only to validate the page against it, but also to activate the standards compliant rendering mode in Internet Explorer 6, Mozilla/Firefox and Opera browsers. If a erroneous or incomplete DTD is used (e.g. you omit the URL part) the standards mode won't be activated and instead the legacy quirks mode will be used.

The Transitional DOCTYPE definition include deprecated tags like font and center, while Frameset DTDs should be used for pages containing frames (i.e. the pages that contain the frameset and frame tags). You can find below the diferent available definitions for HTML 4.01 and XHTML 1.0. They must be placed at the beginning of the (X)HTML file.

HTML 4.01

XHTML 1.0 2

1 Transitional and Frameset DTDs enable in Firefox & Mozilla browsers a third rendering mode called "almost standards" mode.

2 Although proper XML structure requires placing the DTD after the XML prolog, IE6 has a bug that makes the browser fall back to quirks mode if there is anything before the DOCTYPE definition (more about this).

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