gistfile1.html

<!-- Border w/o pseudo elements -->
<style>
  .separator {
   width: 100%;
   height: 1px;
   background: #000;
  }
</style>

<div class="content">
  Hello, world!
</div>

<div class="separator"></div>

<div class="content">
  The world is mine!
</div>

<div class="separator"></div>

<!-- Border using pseudo elements -->

<style>
  .content:after {
   position: absolute;
     bottom: 10px;
   widith: 100%;
   height: 1px;
   background: #000;
  }
</style>

<div class="content">
  Hello, world!
</div>

<div class="content">
  The world is mine!
</div>