To support XML 1.1 in Qt’s QXmlStreamReader and QXmlStreamWriter, several changes would be required, both at the parsing and writing levels:

1. Document declarations:
   1. QXmlStreamReader rejects version 1.1
   2. QXmlStreamWriter allows writing version strings but doesn't change behavior
2. Character set validation
   1. XML 1.1 expands the range of valid characters
3. Entity handling 
   1. Some control characters are permitted when used via character references
   2. Writer needs to escape XML 1.1-only characters as character references
4. Line ending normalization
   1. XML 1.1 parser normalizes U+0085 and U+2028 to LF
   2. Writer should serialize line endings as per XML version
5. Names for elements, attributes etc.
   1. Broader set of Unicode characters is permitted
6. Tests