A new kid on the markup language block is eXtensible markup language. Like HTML, it's an off-shoot of SGML, which is the mother tongue of markup languages. Created to replace HTML, XML puts to use many of SGML's dynamic features to create sites that are even less linear than HTML Web pages. With XML, users can create their own tags, defining and refining them as they go and then sharing them with others; links can point to, and load, numerous pages simultaneously; and links are bi-directional (meaning you can link away to documents - like with HTML - but also link to pages that have linked to the page you're on). So far XML is still in its infancy, though it's doubtful that such a flexible language will go unsupported for long.