> And the way box shadows are implemented on the web is similar to gradients: they use a linear transition.

This is wrong. Yes, linear gradients are everywhere, even in nearly every professional graphic design tool, which is unfortunate since they are almost always the wrong choice. Box shadows, on the other hand, tend to be based on Gaussian blurs, which certainly doesn’t create a linear transition. This is also the way the CSS box-shadows spec say they should work: https://drafts.csswg.org/css-backgrounds-3/#shadow-blur