Probably because the author is thinking about people with poor vision?
IMO, it's poor design for a page to handle accessibility issues that are already built into the browser. (It's trivial to shrink and zoom in a browser.)
I have to wonder what the complaint can be if it's trivial to shrink text - aren't you simply looking at this from the perspective of someone with better vision?
The unspoken assumption here is that you're designing for younger people, which is natural as most designers are younger, but that doesn't make it any better a recommendation than "never go below 18px and 20px is a better lower limit".
Surely it depends on your audience and/or those you wish to attract.
A large part of my recommendation is consistency. For general content sites, it’s not good to be too different from the mean. Font family and size is definitely such a property where you don’t want to deviate too much: twenty years ago, 20px for body type would have been outrageously large; twenty years ago, 11px was definitely on the small side, but not outrageously small as it is now. Conventional sizes have definitely slid upwards, but they’ve peaked in roughly the range 16–20px, and I declare that anything higher than 20px is just too much, pointlessly limiting what you can fit on the screen—any time people go above there, you will see people zooming out because they find it too big.
(And this recommendation is designed for desktop-sized displays; on mobile displays, just use 16px, and certainly don’t go above 18px.)
I am emphatically not assuming designing for younger people; quite the contrary. You will find me saying “don’t go below 16px, you make things harder for many people”. But once you’re in the range 16–18px, going larger just doesn’t help—the people that want to go larger will (or should) already be used to going larger by zooming in, and you’re making life harder for everyone else by preventing the screen from fitting much content at once.
> For general content sites, it’s not good to be too different from the mean.
The mean is decided by those designing sites and they are overwhelmingly younger people - in fact, usually they're among the youngest of all adult workers - and it shows. If sites were designed for the mean amongst society then the move would be to larger fonts.
Personally, I welcome sites that use larger font sizes, I'm not sure why I need to strain my eyes one iota for text in a virtually infinite space. I know I'm not alone (though perhaps for different reasons). Deviating from the mean would work for a substantial part of the population who are currently treated like second class citizens on the web.
> But once you’re in the range 16–18px, going larger just doesn’t help—the people that want to go larger will (or should) already be used to going larger by zooming in,
I've had to show several oldies how to zoom. They do get used to it because they have to.
> and you’re making life harder for everyone else by preventing the screen from fitting much content at once.
Do the people who need this not know how to zoom out?
The banner on this site is too skinny, and the background color is too dull. It doesn't jump out as anything important, and I skipped right over it as well.
A wider banner with more intense background color would go a long way.
I realize that viewing websites on computers is much less the norm than it used to be, but such large text feels really strange.