<h1>Vertical and horizontal center</h1>
<a href="https://vector.cool">vector.cool</a>
<hr>
<div class="wrapper">
  <div class="main v123-xy-center">
    <div class="content">
      Vertical and horizontal center
    </div>
  </div>
  <div class="main v123-xy-center">
    Vertical and horizontal center
  </div>
</div>

.wrapper {
  display: flex;
  justify-content: flex-start;
}

.main {
  width: 300px;
  height: 300px;
  margin: 5px;
  background-color: green;
}

.content {
  width: 200px;
  height: 200px;
  background-color: red;
}

/* CSS Vertical and horizontal center */
.v123-xy-center {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
}