Thank you. Actually, it's because I read on this very page you linked to, that "It is important to remeber that all the transforms act on the XHTML output by the Input Plugin, not on the input file itself", that I decided to ask my question here
I would like to know how this XHTML (the Input Plugin generates) look like. The big picture, really, I guess there is an overall, default structure that's going to be used as a layout for a transform. I have not been able to find that piece of information so far. With the process I'm currently bound to, ebook-convert is used internally, so I am unable to use Calibre's debug mode to inspect the output from the Input Plugin itself meanwhile the transform is happening.