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While this site is sort of useful for looking at screen size, it gives the false impression that users don't scroll. They do. Don't ever show this to anyone that isn't fully aware of this, else you'll fall into the trap of trying to put everything "above the fold", a terrible prison to escape.

Above the fold design is critical from a marketing standpoint. But you're right, everything doesn't have to be above the fold.. if anything the above the fold should be designed to encourage readers to scroll below the fold.

One thing to note is that if you have a site that dynamically re-sizes according to the browser window (by for example using percentages in CSS), then you will physically have to drag your browser bigger and smaller to see how much a user with a smaller display size would see.

Unfortunately ever since X-Frame-Options became popular, this tool has been rather ineffective.