XML was intended as subset of SGML that can be meaningfully parsed without knowing DTD of document in question, which involves removing a lot of weird SGML feat

XML was intended as subset of SGML that can be meaningfully parsed without knowing DTD of document in question, which involves removing a lot of weird SGML features and constraining others. Formally XML is not SGML subset as there are some unimportant and some quite critical incompatible details.