CSS box-shadow takes a horizontal and vertical offset, a blur radius, an optional spread radius, and a color. A positive offset moves the shadow right/down; blur softens its edge; spread grows or shrinks it before blurring.
Inset draws the shadow inside the element's border instead of outside it, useful for pressed or recessed-looking UI elements.
What does spread radius do, exactly?
It expands (positive values) or shrinks (negative values) the shadow's size before the blur is applied, independently of the offset.
Can I stack multiple shadows?
This tool generates a single shadow layer. CSS box-shadow does support comma-separated multiple shadows, but combining several here is out of scope for now.
What does Inset do?
It draws the shadow inside the element's border instead of outside it, for a pressed or recessed look.