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Ok; the classic 640x480 figure is also in pixels, so I'm not sure why the parent commenter would use a different unit to try and draw a comparison.

We’re taking about rendering UI... If I render a div with width=375px and height=667px it will fill the screen on an iPhone 8 because it renders @2x.

There must be a pixel-per-inch ratio in this equation. A 14" monitor running at 640x480 will have around 57 ppi, while iPhone X has like 462 ppi.