Created attachment 65480 [details] test case You can do it now using adoptNode, but there's not reason we shouldn't allow a node to move from one document to another via appendChild. Firefox already allows this and it seems unlikely to me that there is code that depends on throwing an error here. Needing to worry about what document a node is coming from makes multi-document code considerably more complicated (e.g. needs to pass document objects around).
*** This bug has been marked as a duplicate of bug 19524 ***