My pattern library

<style>
#uffa {
  display: block;
  content: '';
  width: 1em;
  height: 1em;
  background-color: white;
}
</style>
<div style="position: relative; display: block; content: ''; width: 6em; height: 4em; background-color: black">
<div class="center" id="uffa">
</div>
</div>