Why Does Absolute Positioning Within Container Require Container to be Relative

Question

It doesn't make sense to me.

If I want a div to be left:20px;top:20px from the container edge, I should be able to do something like position:absolute-within-parent;. Defining the positioning of a child should not require modification of the parent.

At least it seems more modular and decoupling that way. And as a programmer I've been taught to obsess with that.