Matthew Francis6,967 Points
Pixels as a unit
I'm confused does a pixel scale properly with different sizes? Say a screen size for a A is 10 pixels per inch, and for screen size B, it's 20 pixels per inch. if I set a <div> with 40 pixels. It would be 4 inches in screen A; but it would be 2 inches in size B; correct?
Steven Parker179,882 Points
You're basically correct. One additional consideration would be if your operating system offers display scaling (as does Windows 10). This creates a virtual screen resolution that can be different from the actual physical resolution.