Well, that was disgustingly easy. Well, it was once I actually read what Kovid had written about it. Then the hardest part was to find an icon*.
Because of a question in the
Modify ePub thread about sorting the CSS, I decided to try an editor plugin.
It is dumb. It simply adds a toolbar button. Press the button and it sorts each stylesheet on the selectors. Then it presents the differences.
The plugin is attached. I have only tested it with the Qt5 beta code of calibre, but it should be OK with Qt4. I did this for curiosity more than any actual need I have. If there is any real interest, I'll open a thread to discuss it.
* And I don't think I was very successful at that.