<div class="swatch constant"></div>
<div class="swatch mix"></div>

@layer demo {
  .constant {
    background: rgb(255 0 0 / 25%);
  }
  
  .mix {
    background: color-mix(in oklab, red, transparent 75%);
  }
}

@layer demo.support {
  .swatch {
    inline-size: 25vmin;
    aspect-ratio: 1;
    border-radius: 50%;
  }
  
  * {
    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;
    gap: 5vmin;
  }
}