col vertical align code example
Example 1: bootstrap 4 center div
<div class="d-flex justify-content-start">...</div>
<div class="d-flex justify-content-end">...</div>
<div class="d-flex justify-content-center">...</div>
<div class="d-flex justify-content-between">...</div>
<div class="d-flex justify-content-around">...</div>
Example 2: html center image vertically bootstrap
mx-auto mt-auto mb-auto