Animating a CSS Gradient Border

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This little trick for gradient borders is super useful:

.border-gradient {
  border: 5px solid;
  border-image-slice: 1;
  border-image-source: linear-gradient(to left, #743ad5, #d53a9d);
}

Here’s some basic demos from our article on the subject. Sephanie Eckles was sharing around the idea with more detail. Bramus Van Damme saw that and stretched it a bit by adding, then animating an angle to the gradient. Like:

div {
  --angle: 0deg;
  /* … */
  border-image: linear-gradient(var(--angle), green, yellow) 1;
  animation: 10s rotate linear infinite;
}

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

But wait! That’s not actually going to animate as-is. The browser doesn’t know that 360deg is an actual angle value, and not just some random string. If it did know it was an angle value, it could animate it. So, tell it:

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

See Bramus’ article for the demos there. Bonafide CSS trick. I can’t wait for more support for @property (Chrome only, as I write), because it really unlocks some cool CSS trickery. Animating numbers visually, for example.

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