@JSWolf, I either did not know about that or had forgotten. Thank you!!
@ApK, EW.
But seriously, a WYSIWYG tool is still much better.

After all, if the Kindle tells me that the Calibre conversion mucked things up, I still have to outlay money for an editor in which to fix the problem. I know of no free mobi editor on the market. (Unlike Sigil.)