I usually use a max-width of 40rem and add 1rem padding for small screens/windows to keep off the grass, as it were. That anchors the width to the base text size so the line length makes basic sense, no matter what the user font settings might be.
Yes, I agree. I don't like the max-width setting; it should use the width of the window that the user has set. Same with font sizes, and usually, also colours. (Sometimes CSS is helpful (if not overused), but for many documents it will be much better to avoid CSS.) CSS is often what makes web pages look bad.
Does your browser have sidebars? You can add sidebars if you want to reduce the width of the document, I suppose. (There is bookmark sidebar, history sidebar, and web developer sidebar; what I think is missing is the table of contents sidebar, which would list the <h1>, <h2>, etc in the current document. Another thing that might be wanted is split-screen mode, I think.) I personally do want a full-screen line of text, although I suppose some people do not want.
