Answers for "css border bottom length of text heading tag"

border bottom under text only

li {
	a {
		    width: auto;
			display: block;
			height: auto;
            &:hover {
            	border-bottom: 3px solid red;
            }
    }
}

border-bottom smaller div

div {
  width   : 200px;
  height  : 50px;   
  position: relative;
  z-index : 1;
  background: #eee;
}

div:before {
  content : "";
  position: absolute;
  left    : 0;
  bottom  : 0;
  height  : 1px;
  width   : 50%;  /* or 100px */
  border-bottom:1px solid magenta;
}