Understanding the Wireless Markup Language
WML is based on XML, a markup language that has garnered enormous support due its ability to describe data (HTML, meanwhile, is used to describe the display of data...a big difference). While HTML predefines a "canned" set of tags guaranteed to be understood and displayed in a uniform fashion by a Web browser, XML allows the document creator to define any set of tags he or she wishes to. This set of tags is then grouped into a set of grammar "rules" known as the Document Type Definition, or DTD. As mentioned earlier, the DTD used to define WML is located at:
http://www.wapforum.org/DTD/wml_1.1.xml
If a phone or other communications device is said to be WAP-capable, this means that it has a piece of software loaded onto it (known as a microbrowser) that fully understands how to handle all entities in the WML 1.1 DTD.
The first statement within an XML document is known as a prolog. While the prolog is optional, it consists of two lines of code: the XML declaration (used to define the XML version) and the document type declaration (a pointer to a file that contains this document’s DTD). A sample prolog is as follows:
<xml version='1.0'>
<!DOCTYPE wml PUBLIC "-//WAPFORUM//DTD WML 1.1//EN" "http://www.wapforum.org/DTD/wml_1.1.xml">
Following the prolog, every XML document contains a single element that contains all other subelements and entities. Like HTML all elements are bracketed by the
<>
and
</>
characters. As an example: <code><element>datadatadata</element></code>. There can only be one document element per document. With WML, the document element is <code><wml></code>; all other elements are contained within it.
The two most common ways to store data within an XML document are elements and attributes. Elements are structured items within the document that are denoted by opening and closing element tags. Elements can also contain sub-elements as well. Attributes, meanwhile, are generally used to describe an element. As an example, consider the following code snippet:
<!-- This is the Login Card -->
<card id="LoginCard" title="Login">
Please select your user name.
</card>
In the code above, the card element contains the id and title attributes. (On a side note, a comment in WML must appear between the
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