If you stumble upon a website that makes the stylistic choice that text should take the whole width of your whole screen, you can toggle the "Reader view" feature in Firefox to put it back into a more usual format (F9 on Windows).

If you use Chrome-based browsers, there's bookmarklets (bookmarks with a `javascript:` URI)

    javascript:(function(){var bod = document.getElementsByTagName("body")[0]; bod.style.margin = "40px auto"; bod.style.maxWidth = "40vw"; bod.style.lineHeight = "1.6"; bod.style.fontSize = "18px"; bod.style.color = "#444"; bod.style.padding = "0 10px"; })();