For Wicket 1.2.x users The Wicket 1.2.x version of this page can be found here.

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

protected final void configureResponse() {
	super.configureResponse();
	getResponse().setContentType("text/html");
}

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