If you are enumerating the elements in the document fragment which shall be pasted into another document, their root document is normal to be null, because the root document calculations are based on the document collection elements and the corresponding to the elements associated layout boxes.
However, after the insertion of the fragment in the new document, the root document of the inserted DocumentElement objects should be the document where they are inserted to. There isn't a way to change the document owner of a document element and this is by design.
I hope this information is helpful.
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