WML overview

WML is based on the eXtensible Markup Language (XML) and was developed by the Wireless Application Protocol (WAP) Forum.

WML, like HTML, is a type of markup language used for formatting content in mobile devices. WML is very similar to HTML except for some key differences:

Since WML is based on XML, it is a very strict language. All tags must be in lowercase and they must all be closed in order for the code to be well-formed and run correctly.
The Wireless Markup Language is a page description language that describes how WAP content is presented to the user. With WML you can display information on a mobile phone, give the user input options, and specify how the user agent should respond when the user activates a user interface function or presses a key.

WML reference