Border length smaller than div width?

I have following code

div {
   width:200px;
   border-bottom:1px solid magenta;
   height:50px;   
}

DEMO

The div width is 200px so border-bottom is also 200px but what should I do if I want border-bottom only 100px without changing div width?

You can use pseudoelements. E.g.

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;
}

<div>Item 1</div>
<div>Item 2</div>

No need to use extra markup for presentational purpose. :after is also supported from IE8.

edit:

if you need a right-aligned border, just change left: 0 with right: 0

if you need a center-aligned border just simply set left: 50px;

Another way to do this (in modern browsers) is with a negative spread box-shadow. Check out this updated fiddle: http://jsfiddle.net/WuZat/290/

box-shadow: 0px 24px 3px -24px magenta;

I think the safest and most compatible way is the accepted answer above, though. Just thought I'd share another technique.