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Handheld Device Markup Language is used to format content for Web-enabled mobile phones. HDML is Openwave's (formerly known as phone.com) proprietary language, which can only be viewed on mobile phones that use Openwave browsers.

HDML came before the WAP standard was created. It uses Openwave's Handheld Device Transport Protocol (HDTP), instead of WAP.

Phones access HDML sites the following way:

Once the URL is typed into the phone, the phone sends the request to Openwave's UP.Link gateway. The gateway sends a HTTP request to the Web server. The Web servers returns the page via HTTP back to the Openwave UP.Link gateway. The gateway sends the data via HDTP to the wireless carrier's network and down to the phone.

HDML and the Openwave gateway are most popular throughout North America. In Europe, WML and the Nokia WAP gateway and browser are the emerging standard. However, some versions of Openwave browsers do interpret basic WML.

To serve HDML pages, administrators must add the text/x-hdml mime type to their Web servers.

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