To accomplish this you need to override the WebPage.getMarkupType() method in your page class.

For instance, the default:

public String getMarkupType()
{
	return "html";
}

can be overridden to be something like:

public String getMarkupType()
{
	return "xhtml";
}

See the source code for the Alternative Markup (XML) example for a more complete demonstration of this concept.