Indent list when pressing TAB key

I create an unordered list in an iframe with designMode=on using
  editor.contentDocument.execCommand('insertunorderedlist',false,null);

When pressing the TAB key I want to indent the current list item and 
make it a sublist item.
This used to work in 1.9a6pre Gecko/20070627, however it does not in 
current versions. Instead the next element (outside the iframe) will be 
focused. Is there a way to get this behaviour back?

Thanks
Daniel