index.html

<div class="wrapper">
  <div class="vertically-centered">
    Small or large amount of content.
    </div>
</div>
    
<style>
  .wrapper {
    /* make this as tall as you want */
    height: 500px;
  }
  .wrapper:before {
    height: 100%;
  }
  .wrapper:before,
  .vertically-centered {
    /* The height here is dependant upon content */
    display: inline-block;
    vertical-algin: middle;
  }  
</style>