<div class="vert-center horiz-center full-page">
  <div class="block"></div>
</div>
.block {
  height: 100px;
  width: 100px;
  background: red;
}

* {
  box-sizing: border-box;
}

.full-page {
  border: 10px solid black;
  height: 100vh;
  width: 100vw;
  position: fixed;
  top: 0;
  left: 0;
}

.vert-center {
  align-items: center;
  display: grid;
}

.horiz-center {
  justify-items: center;
  display: grid;
}
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