While most XML folk think of DOCTYPE declarations as the way to connect your DTD to your document, Microsoft has found more uses for them in Internet Explorer 5.0 for the Mac.

In "Use the Right Doctype", Eric Meyer shows how the Macintosh Internet Explorer uses DOCTYPE declarations to specify application behavior, in particular standards conformance or backwards compatibility, in addition to identifying which type of HTML is used in a given document. A declaration for Strict HTML yields browser behavior that is extremely conformant, while a declaration for Transitional HTML (or no declaration) yields more relaxed behavior.

It appears to be a unique innovation that supports both backward compatibility and standards.

It isn't entirely clear whether the features left out of the Strict DTD (notably frames) will still be supported when developers use the Strict DOCTYPE to ensure compliance, though it seems that adding support for XHTML 1.0 DTDs - and assuming strict compliance for those documents - might give this technique a wider reach. (Currently IE5 Mac provides some extra flexibility by testing whether or not a URI is provided.)

This usage of DOCTYPE declarations is rather different from the 'resolving' public identifier questions that have been raised lately, but brings up another level of XML processing beyond the parser: application behavior.