A markup language loosely modeled on Hypertext Markup Language (HTML) that is optimized for wireless Internet access by handheld information appliances such as Personal Digital Assistants (PDAs), pagers, and cellular phones. These devices have limited size displays and require that hypertext information be formatted accordingly. Handheld Device Markup Language (HDML) is currently used by wireless carriers such as AT&T, Ameritech, and Bell Atlantic. Its current version is HDML 2.0.
HDML is for devices with displays of 4 lines by 20 characters or smaller. It is not intended as a means of delivering standard Web content to these devices - a standard HTML page simply could not be reformatted to fit on such a device. Instead, HDML provides a new way to deliver information to handheld wireless devices by using wireless Internet Protocol (IP) instead of traditional circuit-switched cellular communication systems. HDML is thus not a subset or scaled-down version of HTML but a new markup language specifically designed for these devices. Another reason not to use scaled-down HTML for these devices is that HTML requires a user interface that allows actions such as clicking links, which is not available for handheld devices.
HDML uses its own navigation model, which is based on the filing card metaphor. Cards are grouped into “activities,” which facilitate the development of HDML-based applications. The user can enter information and share it among cards using variables, and the user can parameterize cards so that a family of cards differing only in the value of a variable can be stored more efficiently in the limited-size cache of these devices. You can use variables to create forms for entering information in HDML applications. HDML currently does not support scripting, branching, or conditional statements.
HDML is likely to be used to deliver a broad range of time-sensitive information to handheld wireless devices, including appointments, weather information, stock quotes, white pages lookups, inventory lookups, and catalog-pricing lookups. Scripts can be developed to extract this kind of information from the databases in which it resides and format it into HDML cards, in much the same way that Perl can be used to write scripts to access database information stored on UNIX servers, or that Active Server Pages (ASP) can be used to build Web-based applications that connect to Structured Query Language (SQL) databases.