By-the-way in Safari Technology Preview 11.2 the minimum font size for all sites is 9 but my setting is 14 and I have the site settings for FineTunedMac set to a Page Zoom of 150%. (My eyes are not as young as they used to be.)
Wow, that's major! (The thought of declining vision is my worst nightmare; the only doctor I ever see is my ophthalmologist; take care of your eyes.)
Well, I cleared my font cache, but with no joy, so I booted into safe mode where FTM looked normal, at which point I decided to do a clean install rather than undertake what promised to be a difficult troubleshooting task, and my FTM screen is back to what I expect to see. (My deuced Mac(hina) had been sluggish for a while, but macOS wouldn't allow me to run the 10.12.6 Combo...even in safe mode.)
The minimum font setting in Safari 11.0.3 prefs is 9, but as respects FTM, 9 and 10 don't work at all, and 11 works only in some instances...curious.