<div class="container">
  <span class="demo center">Thing that should be centered on the page</div>
</div>

.container {
  width: 100vw;
  height: 100vh;
  display: grid;
  align-items: center;
  justify-content: center;
}

.center {
  margin: auto
}

.demo {
  background-color: gray;
  color: white;
  padding: 0.25em;
}