This section describes the functional capabilities which this document type supports. There are three categories of functionality supported: basic bookmark exchange between browsers, data storage for advanced Internet resource management tools, and simplicity in extending the DTD if needed for specific applications.
XBEL instances must be able to describe sufficient data to represent the bookmarks of all major Internet browsers. It must be possible to convert browser-specific bookmark data to XBEL in a lossless manner, though specific conversions may remove data for application-specific reasons. It is especially important to consider privacy issues when exchanging bookmark data.
Conversion from XBEL to a browser-specific format may lose information when the data originates from a browser supporting bookmark features not available in all browsers. It is expected that software implementing the conversion be able to warn the user if conversion will cause the loss of information, as appropriate.
XBEL must be able to support interchange requirements for applications not currently implemented as part of typical Internet browsers, including (but not limited to!) application-specific preference and history information which only pertains to specific bookmarks, metadata information, and alternate sources or formats for the documents.
It must be possible for applications to operate on subsets of the information stored in an XBEL instance without affecting private information stored by other applications. Application-specific data stored in an XBEL instance may be simple text or may be heavily structured.
Some ability to extend the document type definition is required to encourage reuse of the existing design. Due to the use of XML, only a minimum of inherant flexibility is required, as new document types may be formed using namespaces or by allowing the use of well-formed but possibly invalid markup [BHL98].