The syntax <% ... %> is not valid XML. It is used by some templating languages such as JSP and ASP to indicate processing that should be applied to generate portions of an XML or HTML page. Such pages should not be delivered direct to the browser, they should first be processed by the appropriate language processor, e.g. JSP or ASP. This will happen automatically if you get the server configuration right and use the correct file extensions.
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference