(See the sections on CSS and XSL below for more on formatting XML.) Separation of content and presentation is a key concept inherited from SGML. A lot of the programming is already done for you ...

The Root Cause of the Problem The ampersand character has special meaning in SGML, HTML, and XML markup languages. If you wish to use it literally, you must “escape” it.

Unlike SGML, the XML DTD was always optional, because the framers of XML knew there was enormous value in documents that were merely well-formed, even if not valid with respect to a DTD.