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