<aside>
  <h2>Aside</h2>
</aside>
<main>
  <h2>Main</h2>
</main>
@import url("https://fonts.googleapis.com/css2?family=Exo:wght@600&display=swap");

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

body {
  width: 100vw;
  min-height: 100vh;
  font-family: "Exo", Arial, sans-serif;
  background-color: #557;
  display: flex;
  flex-wrap: wrap;
  gap: 20px;
  color: #fff;
}

aside,
main {
  display: flex;
  justify-content: center;
  align-items: center;
  background-color: #dd3433;
  font-size: 2em;
  text-shadow: 0 2px 2px rgb(0 0 0 / 0.5);
}

main {
  background-color: #478ef0;
}

:root {
  --aside-w: 220px;
  --gap: 20px;
}

aside {
  flex: 1 0 var(--aside-w);
}

main {
  flex: 1 1 calc(100% - var(--aside-w) - var(--gap));
}

External CSS

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External JavaScript

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