Font size is not so much about design but usability, but this goes hand in hand. When a size is too small for a good portion of readers to read comfortably, it's bad usability. Nobody should need to increase the size of the regular font on any website to read it comfortably. It's certainly more acceptable for those who for some reason prefer to read smaller fonts to zoom out a notch than to require all those who can't use the site normally due to too small a font to make adjustments so that they can even use the website. Thank your lucky stars if your vision is so superb that it affords you to read increasingly smaller text every year at the same level of comfort. This is definitely not the norm in the human population anywhere, and I'm pretty sure that the Codex is not comprised entirely of eagle-eyed mutants.