<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <div class="outer">
      <div class="inner">
        <div class="box">hello</div>
      </div>
    </div>
  </body>
</html>
* {
        margin: 0;
        padding: 0;
      }

      html,
      body {
        width: 100%;
        height: 100%;
      }

      .outer {
        width: 400px;
        height: 100%;
        background: blue;
        margin: 0 auto;

        display: flex;
        align-items: center;
      }

      .inner {
        position: relative;
        width: 100%;
        height: 0;
        padding-bottom: 50%;
        background: red;
      }

      .box {
        position: absolute;
        width: 100%;
        height: 100%;
        display: flex;
        justify-content: center;
        align-items: center;
      }

External CSS

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External JavaScript

This Pen doesn't use any external JavaScript resources.