That's an implementation detail of the user agent, not anything to do with the website.
The point is "your user agent should make this readable by default, somehow". If it chooses to do so by way of a CSS style sheet, that's the user agent's prerogative, but the standard doesn't care how it goes about rendering a style-free page.
You can say "should make this readable" and talk about "the standard", but in real life you've chosen this specific stylesheet (or its 2 or 3 variations), which effectively hasn't changed since the beginning of the Web, for 99.9% of the times someone looks at your webpage as your stylesheet because it's already preloaded as opposed to choosing a different stylesheet because it looks better or some other criteria.
https://chromium.googlesource.com/chromium/src/third_party/+...
https://github.com/WebKit/WebKit/blob/main/Source/WebCore/cs...
https://searchfox.org/mozilla-central/source/layout/style/re...