Answers for "how inset box shadow works"

box shadow inset

/* offset-x | offset-y | blur-radius | color | inside */
box-shadow: 0px 1px 3px gray inset;

inset boxshadow

.shadow {
   -moz-box-shadow:    inset 0 0 10px #000000;
   -webkit-box-shadow: inset 0 0 10px #000000;
   box-shadow:         inset 0 0 10px #000000;
}