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WML stands for Wireless Markup Language and denotes a markup language that was specifically developed to display mobile websites. WML is a subset of HTML, but also includes elements that have been added only for display on mobile devices. WML is part of WAP (Wireless Application Protocol) which enables and controls the exchange of data between mobile phones and web servers. The markup language is a competing format to XHTML Mobile Profile and is specified through document type definitions (DTD) as a subset of HTML.
WML was developed by the WAP Forum in 1998 and was initially available as version 1.1. In 2001, WML version 2.0 was introduced but it was never broadly used. This is because a little while later, WAP 2.0 was presented and that protocol used XHTML Mobile Profile. The current version of WML is 1.3 and is nevertheless being utilized by many mobile websites. WML has been specifically designed to allow the display of text-based HTML documents and at the same time the display of graphics and other media.
A WML file is structured differently than an HTML file and includes tags or elements which have been developed only for display on mobile devices. WML files are organized by decks and cards. A deck can be accessed via a mobile URL which contains several cards. The cards are the screens that the user can see. Navigation is not handled via URLs, as is the case on the Internet, but via cards.  An example of a WML file, the deck, would look like this:
<?xml version="1.0"?> <!DOCTYPE wml PUBLIC "-//WAPFORUM//DTD WML 1.1//EN"
<card id="main" title= "The first card">
An example of a WML screen.
<card id="sub" title= "The second card">
An example of another WML screen.
Importance for SEO
With the increasing use of mobile devices, the optimization of mobile websites is becoming increasingly important. Depending on the device and the user requirements, varying factors must be taken into account. Files in WML format must be formatted accordingly and should contain correctly edited source code. The code as such can be checked using a validator.  The structure of such files should be kept simple to assist user navigation. Formatting for the different bots that use search engines to read and index mobile content must not be neglected.
A mobile version of a website in the form of responsive designs is recommended in many cases to cover the field of Mobile SEO and give users the ability to access content via smartphone as well. Several technical factors, such as load time, encoding, or the display of text and images have to be taken into account, as well as an optimization of semantic factors with keywords.