<input type="checkbox" />
<div></div>

<footer>
  <p>Demo 4/4 for <a href="https://brm.us/animated-gradient-border" target="_top">https://brm.us/animated-gradient-border</a></p>
  <p>Don't see an animation? That's because your browser does not support <code>@property</code>.<br />Check out <a href="https://codepen.io/bramus/pen/XWMwPgO" target="_top">this forked version</a> which includes a fallback for your browser.</p>
</footer>

div {
  --angle: 0deg;
  width: 50vmin;
  height: 50vmin;
  border: 10vmin solid;
  border-image: conic-gradient(from var(--angle), red, yellow, lime, aqua, blue, magenta, red) 1;
  
  animation: 10s rotate linear infinite;
}

@keyframes rotate {
  to {
    --angle: 360deg;
  }
}

@property --angle {
  syntax: '<angle>';
  initial-value: 0deg;
  inherits: false;
}















input[type="checkbox"] {
  position: fixed;
  top: 1em;
  left: 1em;
}

input[type="checkbox"]:after {
  content: 'Toggle Fill';
  white-space: nowrap;
  padding-left: 1.5em;
}

input[type="checkbox"]:checked + div {
  border-image-slice: 1 fill;
}

html {
  background: #fff;
}

body {
  margin: 0;
  padding: 0;
  height: 100vh;
  width: 100vw;
  display: grid;
  place-items: center;
}

footer {
  text-align: center;
  font-style: italic;
}