The problem is you're imposing your own idiosyncratic preferences on strangers. Your "improvements" cost valuable screen space. I paid good money for my screen space and I don't appreciate designers unilaterally wasting it because they personally think it looks better. The decision of how much (if any) space to spend on padding should be made by the user.
The designers' job is good usability. Limiting line length is important, humans cannot deal well with extremely long lines of text. This is not a matter of taste.
There is no objective definition of "extremely long". Whatever value you choose will be incorrect for some readers. Therefore the correct place to set it is in the web browser's preferences (or the window manager if you prefer).
Useful things to know, but how do they undo a designer wasting my screen space? If you're going to tell me to learn something, it should be CSS. At least Firefox can still be configured to follow its traditional role as "user agent" and allow me to override the bad decisions of designers, by setting toolkit.legacyUserProfileCustomizations.stylesheets and editing chrome/userContent.css . But it would be convenient if more people used the default CSS like danluu.com .
Not sure, as I don't have any wasted screen space, due to the techniques mentioned above. On really bad sites I try reader view first, View/Page Style/No Style (on Firefox) second, and if that doesn't work I close the tab. Pretty rare though.
A website should be readable, even if the browser is set to fullscreen. It could be that the user is on a kiosk system where she can't resize the window or doesn't want to, just to improve legibility.
The problem is you're imposing your own idiosyncratic preferences on strangers. Your "improvements" cost valuable screen space. I paid good money for my screen space and I don't appreciate designers unilaterally wasting it because they personally think it looks better. The decision of how much (if any) space to spend on padding should be made by the user.