WML (Wireless Markup Language) is an XML language used to specify content and user interface for WAP devices such as smartphones. WML documents are XML documents that validate against the WML (currently version 2.0) DTD (Document Type Definition). You can use the W3C Markup Validation service to validate WML documents. For example, the following WML page could be saved as “example.wml”:

   <?xml version="1.0"?>
   <!DOCTYPE wml PUBLIC "-//PHONE.COM//DTD WML 1.1//EN"
      "http://www.phone.com/dtd/wml11.dtd">
   <wml>
     <card id="main" title="First Card">
       <p mode="wrap">This is a sample WML page.</p>
     </card>
   </wml>

WML pages are stored on a web server. They are accessed by a WAP gateway, which sits between mobile devices and the World Wide Web, passing pages from one to the other much like a proxy. This translates pages into a form suitable for handheld devices.

WML is a lot like HTML in that it provides navigational support, data input, hyperlinks, text and image presentation, and forms. A WML document is known as a “deck.” Data in the deck is structured into one or more “cards” (pages)—each which represents a single interaction with the user. Mobile devices are moving towards support for greater amounts of XHTML (Extensible HTML) and even standard HTML as processing power in handsets increases. It is likely WML will be phased out in the future.