The one counter argument to this that I would accept is that users don't know about font settings. But the proper solution if for browsers to make it easier those settings rather than every single website having to add font controls.
Drawing a line can be hard, but clearly the basic font should be contrast-y enough so that most people can read it, no? The user can control the defaults of many things on their devices, but we shouldn't require them to change settings to have a good experience. We already customize websites a lot.
I think it depends on the goals of the print version, but people spend a lot of time picking fonts and styles so things look good on a screen. I take this as saying you should supply the same type of attention to your print style sheet.
I can read my code just fine.
For some harder C code (implementing `malloc` for example) my preferred debugging method was printing out the code and debugging it with a pen between classes.
I eventually realized being inside an editor gave me too much attraction to thrash with "just one more quick iteration & debug run..." and could actually end up more time consuming than being forced to reconcile my mental model with what I had already written by me double checking my assumptions.
Interestingly, some of my peers started picking up the habit for their own projects and we ended up having small "debugging" sessions by trading print-outs .
The only exception is that the layout of your website requires multiple columns so increasing either font size or page zoom doesn’t work well. For example gcc.gnu.org is challenging for a mobile browser if one prefers larger font sizes. You are fine if you aren’t doing that.
The person making the site however wanted someone to change their default font size
We have to also consider that the person in question is most likely not having a bad experience on every app and website, and that this guy just had a tiny font size on his site
Even this famously tiny fonted site is readable on mobile
Did you use
<meta name="viewport" content="width=device-width, initial-scale=1.0">