Not everything that you can do with a QTextDocument formatting is writable to html. For starters any custom properties set by the application (UserFormat+n), but also some standard ones like text outline. This is because there are only 2(3) formats in the mime data created, html, text, and if enabled, odf. There really should be a native format as well to ensure a copy and paste between QTextEdit widgets always is a proper copy and paste. I mean if ODF can be added to the mime data without a decision from the application developer, surely something a little more direct to Qt can be as well.