A big issue with changing to tagging individual elements is back compatibility.  Besides which, if the font is not bold, I cannot imaging bolding anything other than the definition names.  This also seems to venture beyond 'keep IDLE simple for beginners'.  How about a new option to just bold definition names?

Factoring out the call to set_samples to one place is a great idea that simplifies code and tests.  I did it and the htest dialog worked just fine.  The tests also passed as were, but need reworking.  I am doing that now.  The result will be one FontTest class that tests that each widget sets its var, tests that var setting adds changes and calls set_samples, and tests set_samples.