The simplest way is to add this parameter to the PipelineEngine servlet:

<init-param>
    <param-name>content-type</param-name>
    <param-value>application/xhtml+xml</param-value>
</init-param>

Keep in mind this will not display in Internet Explorer as it doesn't understand XHTML.

Firefox, Netscape, etc. can display XHTML just fine.