Assuming you remember all the class names, which ones
will "work" and
which will not (or, worst case, you have documentation handy).
I'd hope documentation would help with identifying class names and one
could imagine extending the visual editor for the most common ones in
future.
Why would certain ones work and certain ones not? With a stylesheet
solution every class should work... (there would possibly just be
different styling rules for those elements for mobile / print etc.. )