border bottom not over full width code example
Example: how to set border length in css without 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;
}