<section>
  <div id="object"></div>
  <div>Hello World!</div>
</section>

body {
  font-family: Helvetica, Arial, sans-serif;
  font-size: 18px;
}

section {
  height: 100vh;
  overflow: hidden;
  display: flex;
  flex-direction: column;
  /* Align all my children horizontally in the centre. */
  justify-content: center; 
  /* Let's also vertically align in the centre all items added to the flex--not just one. */
  align-items: center;
}

#object {
  height: 100px;
  width: 100px;
  background: red;
  align-self: center; /* Just vertically aligns me ;-) */
}