The extra markup will only be added if you use the Rich Text tab of the editing dialog. Your own markup will be preserved verbatim if you:
- directly edit the label (i.e. via double-click or pressing F2)
- add it to the line-edit of the text property
- use the Source tab of the editing dialog
EDIT:
To answer the question of whether the full markup is strictly necessary: no, not really. According to the Qt docs on the Supported HTML Subset, it is compliant with the HTML 4 spec. So, as with any reasonably competent html-rendering engine, it should have no difficulty dealing with an html document containing only the fragment <strong>My bold label!</strong>
.