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

.container {
  width: 100vw;
  height: 100vh;
}

.center {
  max-width: fit-content;
  margin-inline: auto;
}

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