From Seo Wiki - Search Engine Optimization and Programming Languages
Some of the more elabourate features of GuideML are referred to as "gadgets" because they perform functions beyond straight-forward formatting. These include the ability to format a list of user IDs into a drop-down list with the option of opening in the same or a different window; and the famous
<VIEWER/> tag, which displays as the username of the person viewing the page, if they're logged in.
A proposal for a standardized "GuideML 1.0" was under discussion, and would have involved a specific subset of the currently supported tags being declared "official" parts of the language. This would have been an important change of approach, in that the current parser lets through any unrecognized tags, as long as they are XML-compliant, meaning that any XHTML understood by the viewer's browser is implicitly part of GuideML.