Nodes from external documents should be cloned using
importNode() (or adopted using
adoptNode()) before they can be inserted into the current document. For more on the
ownerDocument issues see the W3C DOM FAQ.
Firefox does not presently enforce this rule (it did for a while during the development of Firefox 3, but too many sites break when this rule is enforced). We encourage web developers to fix their code to follow this rule for improved future compatibility.