I haven't upgraded to Safari 10 yet (although I may install it in one of my clones to play with it) but I wonder if the font size issue is addressable with TinkerTool 6 for Sierra or older versions for previous operating systems. TT has a Safari tab that allows one to set default fonts and sizes but I don't know if it works with Safari 10.

Any brave souls out there who want to try it?

Jon

macOS 11.7.10, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365