I have found the solution here: HTML5 JSPX template
Using the template from that page, this JSPX works as expected:
<?xml version="1.0" encoding="utf-8" ?>
<jsp:root xmlns:jsp="http://java.sun.com/JSP/Page" version="2.0">
<jsp:directive.page contentType="text/html" pageEncoding="utf-8" />
<jsp:output omit-xml-declaration="true" />
<jsp:output doctype-root-element="HTML"
doctype-system="about:legacy-compat" />
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<script type="text/javascript" src="http://code.jquery.com/jquery-1.10.2.js"><!-- required for FF3 and Opera --><jsp:text> </jsp:text></script>
<script type="text/javascript" src="jqReady.js"><!-- required for FF3 and Opera --><jsp:text> </jsp:text></script>
</head>
<body>
<div id="hello">Empty</div>
</body>
</html>
</jsp:root>