Compound documents

It is possible for a single XML document to contain fragments that are defined in a number of different DTDs or schemas. The ability to do this resolves a number of issues.

To facilitate rendering of complex structures in a browser, it may be necessary to embed HTML elements within a document that does not otherwise conform to this standard. For example, HTML tables (see Chapter 23) are powerful, and well supported by the popular Web browsers, so it would be useful to be able to use HTML elements whenever a table is needed:

This is actually just one case of a general need to match data to the expectations of widely used tools that have a specific purpose, beyond the individual domains that DTDs are associated with. Another example would be a Web crawler searching documents for specific forms of meta-data to improve the classification of sites for a search engine (but see Architectural forms in Chapter 6 for another approach). The XSLT standard also requires this concept to allow the mixing of formatting instructions with target document element tags, and the XLink standard (Chapter 27) uses namespaces to add linking roles to elements from any DTD or XML Schema model.