How to make shadow on border-bottom?

Try:

div{
-webkit-box-shadow:0px 1px 1px #de1dde;
 -moz-box-shadow:0px 1px 1px #de1dde;
 box-shadow:0px 1px 1px #de1dde;
  }

<div>wefwefwef</div>

It generally adds a 1px blurred shadow 1px from the bottom of the box

box-shadow: [horizontal offset] [vertical offset] [blur radius] [color];

The issue is shadow coming out the side of the containing div. In order to avoid this, the blur value must equal the absolute value of the spread value.

div {
  -webkit-box-shadow: 0 4px 6px -6px #222;
  -moz-box-shadow: 0 4px 6px -6px #222;
  box-shadow: 0 4px 6px -6px #222;
}

<div>wefwefwef</div>

covered in depth here