The World Wide Web provides a robust, flexible, and ubiquitous model for information access. The adoption of the WWW as the preferred means of disseminating and accessing information from desktop PCs and workstations has created a demand for access to the same information for other devices. These devices or "alternate platforms" range from voice- and fax-based user agents to low-cost Network Computers to handheld devices such as mobile phones and PDAs.
While the web's infrastructure and protocols fully support these alternate platforms, HTML itself does not. In particular the navigation and display models inherent to HTML collapse when applied to a typical handheld device. However, combining the use of standard web protocols and infrastructure (URLs, HTTP, SSL plus CGI, Perl, commercial web servers) with an alternate but complementary markup language, allows handheld devices to function as full-fledged web clients.
While there are many types, styles, and classes of handheld devices, this specification is useful for a significantly larger class of devices with similar physical characteristics. Those characteristics include:
This specification will use the data-ready mobile phone as an example of the typical handheld device. This specification will also continue to use the term "handheld," recognizing that the term is not inclusive of all devices which would benefit from HDML.
Handheld devices are characterized primarily by a limited display size. A typical display is capable of displaying 4-10 lines of text 12-20 characters wide and may be graphical (bitmapped) or text-only. PDA-style displays are not necessarily included in this handheld device category, although HDML will be useful on those devices as well.
Handheld devices may or may not have a full keyboard and may or may not have a pointing/selection device. As an example, the data-ready mobile phone has only:
Alphanumeric pagers have even fewer keys. Item selection is accomplished through numbered lists or using cursor keys to highlight a choice and then request an action on that item. Full 2-D cursor control through pointing devices such as touch pads, touch screens, roller balls, are rare. A full QWERTY keyboard is also rare. Input is severely constrained when compared to a typical PC or PDA.
Network bandwidth is usually low due to limitations of the network technology or simple economics. The same goes for other resources: memory, processing power, even battery life. All in the name of economics. While some devices do have large amounts of memory or processing power, these devices are the exception. Mass market and consumer-targeted devices will continue to have these constraints for many years.