I made a similar Greasemonkey custom script

[foriequal0]

Thank you for making an awesome Too Wide!. It has eased my eyes countless times.
There were some features that I hoped to have such as persistence (now it does have), but I was shy to contribute to your work.
While I was inspired by your previous version, I've made a Grease monkey custom script for myself.
https://gist.github.com/foriequal0/12794f45f5ef60249e8ba36f978dfaa4
I'm wondering whether you might be interested in some experimental approaches that mine has.

1. It travels the DOM and make a heuristic decision based on getClientRects and text lengths. (although it has some false positive such as a lenovo website)
2. It iteratively clamps the root document to match the target width for complex and poorly responsive documents.
3. The target width is calculated with relative to the text size (in my script, it is 50em), not fixed pixel width.
4. It applies following css to the document

:root {
  max-width: ${width}px;
  position: absolute;
  left: calc(50% - ${width}px / 2);
  transform: translate(calc(${width}px / 2 - 50%));
}`

It makes some difference on some elements that have position: fixed.
position: absolute & left & transform

Too wide!

Page: https://ko.wikipedia.org/wiki/%EA%B7%80%EC%9D%B8_%EC%9D%B4%EB%A1%A0