Tricks like this are self-perpetuating because it teaches users that progress bars are UI meant to model a duration. This is of course impossible to calculate w

Tricks like this are self-perpetuating because it teaches users that progress bars are UI meant to model a duration. This is of course impossible to calculate without a crystal ball, as other posters have pointed out. But since it's what users expect, web developers must create an animation that meets user expectation.