If a site like Wikipedia is too wide, wouldn't you just make your browser window thinner? Mine has never been wider than half the display since I got a widescreen LCD ~ 15 years ago.
Interestingly, before they added the iOS-style fullscreen mode, the "maximise" button on Mac OS X would actually just fit the window to the content, rather than the screen.
A common frustration to those used to Windows or Linux, while the Mac folks were equally confused about why people wanted full-width windows with miles of empty space on either side.