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I came up with a design rule for websites: they should show content well when a browser window is half the screen of a 15 inch laptop (MacBook, or Windows laptop).
Why this rule?
Because that's how I setup my workspace most of the time. I put the browser in one half of the screen and, say, a text editor, in the other half.
Windows makes it very easy thanks to window positioning gestures (drag window title and move it to the edge of screen). On Mac I use Moom utility for that.
Sadly, the design trend of websites is to use lots of white space and favor aesthetics over usability.
Here's just one of many examples. Heroku's website at half screen on 15" Thinkpad x25:
After expanding the window just by a little bit:
I would rather they've hidden the "HEROKU" text (I know what website I'm on, thank you very much) and showed the full navigation bar at half-screen size.
Things are getting worse in that respect. The old Reddit design shows everything on half screen. The new design? You have to significantly resize to see the sidebar.
Can't do much about what other people do, but I am and will continue to test my websites against the half-screen rule.