<div class="wrapper">
  <div class="widget red">
    <div>R</div>
    <div class="bar">
      <div class="indicator"></div>
    </div>
    <div class="value"></div>
  </div>

  <div class="widget green">
    <div>G</div>
    <div class="bar">
      <div class="indicator"></div>
    </div>
    <div class="value"></div>
  </div>

  <div class="widget blue">
    <div>B</div>
    <div class="bar">
      <div class="indicator"></div>
    </div>
    <div class="value"></div>
  </div>

  <div class="swatch">
    <div class="rgb">RGB</div>
  </div>
</div>

<div class="support-warning">
  Sorry, your browser doesn't support Houdini stuff so you won't see the animations. Try Chrome, Edge, or Opera.
</div>

@property --red {
  syntax: '<integer>';
  inherits: true;
  initial-value: 0;
}
@property --green {
  syntax: '<integer>';
  inherits: true;
  initial-value: 0;
}
@property --blue {
  syntax: '<integer>';
  inherits: true;
  initial-value: 0;
}

@keyframes red-fade {
  50% { --red: 255; }
}
@keyframes green-fade {
  50% { --green: 255; }
}
@keyframes blue-fade {
  50% { --blue: 255; }
}

:root {
  animation: red-fade 16s, green-fade 14s, blue-fade 12s;
  animation-iteration-count: infinite;
}

*, *::before, *::after {
  box-sizing: border-box;
}

body {
  display: grid;
  justify-content: center;
  align-items: center;
  min-height: 100vh;
  margin: 0;
  padding: 20px;
  color: #fff;
  background-color: #333;
  background-image: linear-gradient(135deg, #111, #444);
  font: bold 1.25rem/1 'Roboto Mono', monospace;
}

.wrapper {
  display: grid;
  grid-template-columns: repeat(3, 40px) minmax(0, 50%);
  justify-content: space-around;
  gap: 10px;
  width: 600px;
  max-width: calc(100vw - 40px);
  padding: 20px;
  border-radius: 20px;
  background-color: #0006;
}

.widget {
  display: grid;
  grid-template-rows: auto max-content max-content;
  justify-items: center;
  gap: 10px;
}

.bar {
  display: grid;
  width: 40px;
  height: 257px;
  border-radius: 5px;
  border: 1px solid #fff;
  background-color: #fff;
}

.indicator {
  align-self: end;
  height: 4px;
  margin: -2px -6px;
  border-radius: 2px;
  background-color: #fff;
}

.value::before {
  content: counter(color-value);
}

/* red */
.red .bar {
  background-image: linear-gradient(to bottom, red, black);
}
.red .indicator {
  transform: translateY(calc(var(--red) * -1px));
}
.red .value::before {
  counter-reset: color-value var(--red);
}

/* green */
.green .bar {
  background-image: linear-gradient(to bottom, lime, black); /* lime, not green! */
}
.green .indicator {
  transform: translateY(calc(var(--green) * -1px));
}
.green .value::before {
  counter-reset: color-value var(--green);
}

/* blue */
.blue .bar {
  background-image: linear-gradient(to bottom, blue, black);
}
.blue .indicator {
  transform: translateY(calc(var(--blue) * -1px));
}
.blue .value::before {
  counter-reset: color-value var(--blue);
}

.swatch {
  background-color: rgb(var(--red), var(--green), var(--blue));
  justify-self: stretch;
  align-self: center;
  display: grid;
  justify-content: center;
  align-items: center;
  height: 257px;
  border-radius: 5px;
  border: 1px solid #fff;
  font-size: 3rem;
}

.rgb {
  mix-blend-mode: exclusion;
  color: #fff;
}

.support-warning {
  display: none;
}
@supports not (background: paint(something)) {
  .support-warning {
    display: block;
    position: absolute;
    top: 0;
    right: 0;
    left: 0;
    padding: 20px;
    color: #000;
    background-color: #fff;
    text-align: center;
  }
}