<div style="--size: 40vh"></div>
<div style="--size: 40vh 50%"></div>
<div class="no__size"></div>
<div class="no__size"></div>
@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;
  display: grid;
  grid-template-columns: repeat(2, 40vh 1fr);
  padding: 10px 10px 30px;
  gap: 30px 10px;
  font-family: "Exo", Arial, sans-serif;
  background-color: #222;
  color: #fff;
  place-content: center;
}

:root {
  --color-start: #09f;
  --color-end: rgba(255, 255, 255, 1);
  --size: ;
}

div {
  border-radius: 5px;
  border: 1px solid #efefef;
  min-height: 40vh;
  position: relative;
  background-image: radial-gradient(
    var(--size),
    var(--color-start),
    var(--color-end)
  );
}

.no__size {
  background-image: radial-gradient(var(--color-start), var(--color-end));
}

div::after {
  content: attr(style);
  position: absolute;
  top: calc(100% + 5px);
  white-space: nowrap;
  left: 50%;
  transform: translate(-50%, 0);
}
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