In addition to Bear's answer it's useful to note that on iOS 1pt in a native app is equal to 1px in CSS, and on Android 1dp in a native app is equal to 1px in CSS. So if you have an image and display it at 100x200pt on iOS, 100x200dp on Android, and 100x200px on a web page, they will appear at the same size on the same device. (Also note that CSS has its own pt unit, which is 1.333 (96/72) times larger than a px, however this is different from an iOS pt.)
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