The user should be able to control content width. For example, an unstyled page should show in the user's preferred content width by default, instead of the screen/window width. Pages can then adjust their content based on the preferred content width. Also, it should be possible to set a preferred content width bigger than the screen width, for example if the user happens to prefer horizontal scrollbars on mobile.

This is a pretty significant accessibility issue. Some folks have made userstyles to provide site-specific hacks (see e.g. any of a number of "wide-github" userstyles), but they break regularly.