DTD: Document Type Definitions as far as the eye can see

For something that is completely unused by any web applications, people sure do pay a lot of attention to it.

A document type definition theoretically contains information used by XML parsers to determine whether the XML document is "valid" - contains the correct elements and attributes, can be closed within the same tag (by putting a / at the end of it), and does not contain anything really much more useful than a comma-delimited-values file.

However, the DTD cannot define whether a tag is allowed to close itself WITHOUT using the slash. Such an upgrade to the DTD standard would have inconvenienced a few professional geeks, so instead we altered the whole HTML standard to use the XML slashes instead. This move has meant that those 50 or so application developers have not been inconvenienced, and instead millions of web designers have had to change their code.

Which applications check the document type definition against the XML document to be parsed? Well... none. Yet. But XML and the DTD have only been around for about five years, so it's still early days.