XML in a Nutshell, 3rd Edition

ANY

Very loose DTDs occasionally want to say that an element exists without making any assertions about what it may or may not contain. In this case, you can specify the keyword ANY as the content specification. For example, this declaration says that a page element can contain any content, including mixed content, child elements, and even other page elements:

<!ELEMENT page ANY>

The children that actually appear in the page elements’ content in the document must still be declared in element declarations of their own. ANY does not allow you to use undeclared elements.

ANY is sometimes useful when you’re just beginning to design the DTD and document structure and you don’t yet have a clear picture of how everything fits together. However, it’s extremely bad form to use ANY in finished DTDs. About the only time you’ll see it used is when external DTD subsets and entities may change in uncontrollable ways. However, this is actually quite rare. You’d really only need this if you were writing a DTD for an application like XSLT or RDF that wraps content from arbitrary, unknown XML applications.