Border Beam: CSS Border Animation

Border Beam

A short beam of light travels the edge of a dark box over a faint track, the look from AI and developer tool sites. A narrow conic arc on an @property angle, masked to the edge and given a glow.

Also known as: border beam css, moving border animation, light running around border.

Border beam

Color Base size 16px Dark background Hold hover state

HTML

<div class="bdr-border-beam"><span class="bdr-border-beam-light"></span>Border beam</div>

CSS

@property --bdr-border-beam-a {
  syntax: "<angle>";
  inherits: false;
  initial-value: 0deg;
}
.bdr-border-beam {
  --c: var(--bdr-color, #3b82f6);
  font-size: var(--bdr-size, 16px);
  position: relative;
  display: grid;
  place-items: center;
  width: 15em;
  height: 8.5em;
  border-radius: 1em;
  font-family: system-ui, sans-serif;
  font-weight: 600;
  background: color-mix(in srgb, var(--c) 10%, #0b0f19);
  box-shadow: inset 0 0 0 1px color-mix(in srgb, #fff 12%, transparent);
  color: #f1f5f9;
}
.bdr-border-beam-light {
  position: absolute;
  inset: 0;
  border-radius: inherit;
  filter: drop-shadow(0 0 0.25em var(--c));
  pointer-events: none;
}
.bdr-border-beam-light::before {
  content: "";
  position: absolute;
  inset: 0;
  padding: 0.14em;
  border-radius: inherit;
  background: conic-gradient(from var(--bdr-border-beam-a), transparent 0 74%, color-mix(in srgb, var(--c) 60%, transparent) 86%, var(--c) 94%, color-mix(in srgb, var(--c) 25%, #fff) 98.5%, transparent 99%);
  -webkit-mask: linear-gradient(#000 0 0) content-box, linear-gradient(#000 0 0);
  -webkit-mask-composite: xor;
  mask: linear-gradient(#000 0 0) content-box exclude, linear-gradient(#000 0 0);
  animation: bdr-border-beam-orbit 3s linear infinite;
}
@keyframes bdr-border-beam-orbit {
  to { --bdr-border-beam-a: 360deg; }
}

@media (prefers-reduced-motion: reduce) {
  .bdr-border-beam, .bdr-border-beam *, .bdr-border-beam::before, .bdr-border-beam::after, .bdr-border-beam *::before, .bdr-border-beam *::after {
    animation-duration: 0.01ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.01ms !important;
  }
}

Customizing this snippet

Color: set --bdr-color on the element or a parent. Currently previewing #3b82f6.
Base size: set --bdr-size. Every other dimension is in em, so the whole thing scales together. Currently previewing 16px.
Speed: change the duration on the transition or animation shorthand. Staggered delays are relative, so they keep their rhythm.
Reduced motion: the rule at the end of the CSS turns the motion off for people who asked their OS for less of it. Keep it.

Border Beam

HTML

CSS

Customizing this snippet

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