All well-formed and valid XML 1.0 documents remain well-formed and valid XML 1.1 documents
version attribute in the XML declaration has the value
Many more scripts are allowed as name characters, i.e tag names, attribute names, entity names, ID attribute values, etc.
Most C0 controls (vertical tab, bell, formfeed, etc.) are now allowed in element content
NEL (0x85) is now allowed as white space along with carriage return and linefeed.
All of these cause major interoperability problems and should not be used.