A side-effect of Windows pinning fonts to pixels is that you can't get arbitrary font sizes and scaling. E.g. I had to use “font-size: 82%” in CSS, because at 80% the text looked bad (pinned to the wrong pixel, and possibly became blurry, can't remember for sure). Likewise, zooming in browsers was a mess.
This was ten years ago, so dunno if anything has changed.
Afaik 16px only relatively lately became the ‘default size’ on the web. I'm not even sure that ‘css pixels’ were a thing ten years ago. But it's possible.
I’m not exactly sure either but that is around the time I got started with css and I can still recall a lot of these 16px conversion ratios[1], they must have hade some significance.
Edit: now that I’m thinking about it, I (used to) use Spotlight for the conversions which was introduced in Mac OS X 10.4, released in 2005.
This was ten years ago, so dunno if anything has changed.