<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <div class="center">WOW</div>
</body>
</html>
body {
  background-color: #ddd;
}

.center {
  background-color: orange;
  width: 100px;
  height: 100px;
  position: absolute;
  left: 50%;
  /* center의 부모인 body의 너비 기준에서 50% 이동 */
  top: 50%;
  transform: translate(-50%, -50%);
}

External CSS

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External JavaScript

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