<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>
