Browser defaults, like Discourse defaults, are generally recommendations that are a good starting point for everyone.
In this case, there’s a large amount of advice that recommends 16px as the minimum:
Accessibility Tips: Let Users Control Font Size
Body text should be at least 12 points (pt), or around 16 pixels (px).
Minimum font size? | Accessible Web
While there is no official minimum font size for the web, it is generally agreed upon that 16px for body text is a good starting point. Of course, some text will be smaller and headings will often times be larger. But, the main body text (like what you’re reading right now) should usually be 16px or larger.
Accessibility at Penn State | Font Size on the Web
- For traditional computer monitors, a size of 12pt (=16px) for body is generally recommended for body text (depending on audience).
If we did this for every change we’d have an enormous number of separate “legacy” CSS branches to reason with every time we made an update, which would make maintenance and debugging fairly difficult. It could also mean certain “legacy” CSS would become incompatible with themes and plugins, etc.
It’s much easier to copy and paste a few lines of CSS into a theme for the (likely few) sites that want to revert.