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XHTML
Basically HTML expressed as valid XML. XHTML is intended to be used in the same places you would use HTML but is much more strictly defined, which makes it a lot easier to create software that can read it, edit it, check it for errors, etc.

XML – Extensible Mark-up Language
A standard way of storing and transferring data between programs and over the Internet. They are formatted as text documents, and can be edited by a basic text editor.