<div class="circle"></div>

@layer demo {
  .circle {
    inline-size: 50vmin;
    aspect-ratio: 1;
    border-radius: 50%;
    
    background: Highlight;
  }
}

@layer demo.support {
  * {
    box-sizing: border-box;
    margin: 0;
  }

  html {
    block-size: 100%;
    color-scheme: dark light;
  }

  body {
    min-block-size: 100%;
    font-family: system-ui, sans-serif;

    display: grid;
    place-content: center;
  }
}