It looks totally fine to me (lines are wrapping normally). Are you sure it's not something to do with your own browser?
Not to mention some sites have things like side bars, images or other UI elements that make them more comfortable to user with larger viewports so you'd resize your browser often when you switch tabs.
It would be useful if sites could specify a "body" element that could be resized and aligned however the user wishes without affecting other parts of the site or the browser UI.
I took the aforementioned screenshot on a huge display to illustrate the issue more clearly, but on a garden-variety 1920×1080 monitor — stock Firefox, no extensions that would affect rendering — Fossil wiki's text formatting is still objectively bad.
Do not wire Postel's Law into anything new.