Following this answer, I was able to insert some text with Firefox like this
css
body
{
  -moz-binding: url(foo.xml#bar);
}
foo.xml
<bindings xmlns="http://www.mozilla.org/xbl">
  <binding id="bar">
    <content>
      Hello world
    </content>
  </binding>
</bindings>
However I could not find a way to insert HTML rather text using this method.
 
    