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Augmenting HTML
Adding semantic information to HTML pages is not easy. Historically, various programs have attempted to deal with this problem by using nonstandard tricks, such as hiding data inside HTML comments. However, these comments are awkward and, unlike XML, are not exposed to the object model.
To solve this, the World Wide Web Consortium (W3C) has defined a format for putting XML-based data (data islands) inside HTML pages. Extending HTML through the use of data islands will allow a wide range of applications to use HTML as the primary document or display format, and also to use XML embedded within these documents to hold data.
An HTML page can therefore include, among other things, specific data about the subject of the page. For example, if the page displayed an advertisement for an author's most recent novel, the page can also contain XML data concerning that book, such as its ISBN number, publisher, or suggested retail price. It is not important that this information be displayed, but it is important that this information be accessible and understandable as data.