You can write JSP pages in XML, these are called JSP documents. It's optional. It comes with all the advantages of any XML document, like the possibility to create XML well-formed pages. Everything must be written with a opening and closing tag. For example, you cannot write a scriptlet like this :
You have to write it :
You can read all the gory details in the JSP Specification, JSP.6 JSP Documents.
The document format is not intended for hand coding. Those that have tried can tell you the tales of woe trying to get things to work correctly. You end up with so much markup that you loose sight of what the page is supposed to do in the first place.
Rather, the format is intended as an intermediate format for containers, or where a page might be built automatically as a DOM and flattened to XML format.