<div class="box1">box1</div>
div {
  display: flex;
  align-items: center;
  justify-content: center;
}

.box1 {
  background : #3898E0;
  width :100vw; /* ビューポート幅の100% */
  height: 100vh; /* ビューポート高さの100% */
}

External CSS

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External JavaScript

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