[t0astbread]

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.

[ncmncm]

There are these things called "windows" (not an MS trademark!), and you can have more than one.

[t0astbread]

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.