Two Tone Border: CSS Border Animation

Two Tone Border

Two colors share the border and play tug of war: the split drifts round while each side keeps winning and losing ground. One @property angle turns it and a second percentage moves the split.

Also known as: two color border css, half and half border css, split color border.

Two tone

Color Base size 16px Dark background Hold hover state

HTML

<div class="bdr-two-tone-border">Two tone</div>

CSS

@property --bdr-two-tone-border-a {
  syntax: "<angle>";
  inherits: false;
  initial-value: 0deg;
}
@property --bdr-two-tone-border-s {
  syntax: "<percentage>";
  inherits: false;
  initial-value: 50%;
}
.bdr-two-tone-border {
  --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;
  --two: color-mix(in oklch, var(--c), #f59e0b 85%);
  box-sizing: border-box;
  border: 0.22em solid transparent;
  background:
    linear-gradient(color-mix(in srgb, var(--c) 10%, #0b0f19), color-mix(in srgb, var(--c) 10%, #0b0f19)) padding-box,
    conic-gradient(from var(--bdr-two-tone-border-a), var(--c) 0 calc(var(--bdr-two-tone-border-s) - 3%), var(--two) calc(var(--bdr-two-tone-border-s) + 3%) 97%, var(--c)) border-box;
  color: #f1f5f9;
  animation:
    bdr-two-tone-border-turn 8s linear infinite,
    bdr-two-tone-border-tug 2.4s cubic-bezier(0.45, 0, 0.55, 1) infinite alternate;
}
@keyframes bdr-two-tone-border-turn {
  to { --bdr-two-tone-border-a: 360deg; }
}
@keyframes bdr-two-tone-border-tug {
  from { --bdr-two-tone-border-s: 28%; }
  to   { --bdr-two-tone-border-s: 72%; }
}

@media (prefers-reduced-motion: reduce) {
  .bdr-two-tone-border, .bdr-two-tone-border *, .bdr-two-tone-border::before, .bdr-two-tone-border::after, .bdr-two-tone-border *::before, .bdr-two-tone-border *::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.

Two Tone Border

HTML

CSS

Customizing this snippet

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