Scroll shadows are when you can see a little inset shadow on elements if (and only if) you can scroll in that direction. It's just good UX. You can

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 scroll shadows are when you can see a little inset shadow on elements if (and only if) you can scroll in that direction. You can actually pull it off in CSS, which I think is amazing and one of the great CSS tricks. Except… it just doesn’t work on iOS Safari.

Scroll Shadows With JavaScript