border-bottom
Syntax:

border-bottom: border-bottom-width | border-bottom-style | border-bottom-color


<style type="text/css">
.demo1 {
   border-bottom: 10px solid #FF0000;
   background: #00FF00;
   width: 200px;
   height: 50px;
}
</style>
<div class="demo1"></div>
<style type="text/css">
.demo2 {
   border-bottom: 10px dotted #0000FF;
   background: #00FF00;
   width: 200px;
   height: 50px;
}
</style>
<div class="demo2"></div>