I can’t change the box shadow distance unit type. The option to change it from “px” is grayed out. I am not able to select any other unit type (ONLY for “distance”). I have tried typing in the box, “20vw” or “20%” to no avail.
This is IMO the best answer you can get but there are many other resources if you look around and want do dive deep into W3C specs to have discussion why. There are another ways to achieve this underlines as using pseudo classes :before and/or :after etc…