True, the wired community is always expanding the HTML tag set, but because the browser manufacturers insist upon backward-compatibility, even the newest tags must adhere to strict rules and must be used correctly. In contrast, XML lets you decide how data should be indexed, manipulated, and displayed. Let's say you want to create a set of tags that match the fields in your inventory database. HTML has no way of using this data, because for the most part, it only governs a page's appearance. With XML, every department in your company could use the inventory tags you create to generate pages on an intranet, showing such data as stock on hand, on order, and out the door.
Users can then access it however they choose.
Apart from that restriction, though, XML is about as permissive as markup languages get. Once you've written a set of tags, you can use them anywhere, and other users can simply copy and paste them into their own pages. The word processor can only set up a document's appearance, but the database can actually manage content and determine what a given piece of data means. You can use XML to create Web pages that actually transact with other documents and process content. For instance, you might want to write an embedded XML program that locates and downloads other XML pages, extracts sales data, and then processes those numbers into meaningful information.