I have never used <small>, <large>, <x-small>, etc. I've always just styled font-sizes for a special <p>, or <div>, or <span>.
Now I am facing a project that could require Hundreds of spans (my very least favorite dessert), and I thought I would try those tags instead.
I often style <em> and <strong>, and it seems I can also style <small>, <large>, etc. These display fine in ADE for PC and Calibre reader. I can set <small> to be .75em; bold; , and <large> to be 1.5em, italic --- and they work.

Stupid question of the day (but better safe than sorry) --- will other software and reading devices honor this styling?
Thanks