<div id="content">
  <p>コンテンツ</p>
</div>
#content{
  /* 装飾関連 */
  display:flex;
  align-items:center;
  justify-content:center;
  background:#00f;
  margin:0 auto;
  
  width:50vw;
  /*
    50vw = ウィンドウサイズの横幅半分の大きさ
  */
}

External CSS

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External JavaScript

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