I think the complaint was that lines aren't capped somewhere at 70-80 characters but rather fill the entire remaining space in the viewport which can make the text hard to read.
I see where you're coming from but resizing the browser to your preferred text width would mean decreasing the space for elements like the tab bar or other "not-in-body" UI elements even though there's still screen space available. I'm not sure that's something most people would want.
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.
Right but then it becomes a mess to find anything if you have to look in multiple tab bars or you have a lot of windows which could also become hard to manage. My point is, while your approach works for you it's probably not useful or intuitive to enough people to justify designing your website around it.
Yes. I'm not a stickler for 70—80 characters; IMHO the ~200 character lines on HN are excessive, but I can cope. You know, Postel's law: be conservative in what you do, be liberal in what you accept from others.
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.