center a block element

  display: inline-block;
  position: relative;
  /* Move the element to the right by 50% of the container's width */
  left: 50%; 
  /* Calculates 50% of the element's width, and moves it by that */ 
  /* amount across the X-axis to the left */
  transform: translateX(-50%);

margin-left: auto;
margin-right: auto;