<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" xmlns:html="http://www.w3.org/1999/xhtml" style="font-size:0"> <html:span style="display:inline" flex="1" /> <!-- causes block wrapping --> <box flex="1" /> <label style="font-size:16px" value="hello world" /> </window>