Square to Circle Morph: CSS Keyframe Animation

Square to Circle Morph

Rounds off into a circle while it turns half a revolution, holds, then squares up again on the way round. border-radius interpolates happily from an em value to 50 percent, so one property does it.

Also known as: css square to circle animation, border-radius morph animation, shape morph css.

Color Base size 16px Dark background

HTML

<div class="kfa-square-to-circle-morph"></div>

CSS

.kfa-square-to-circle-morph {
  --c: var(--kfa-color, #3b82f6);
  font-size: var(--kfa-size, 16px);
  display: grid;
  place-items: center;
  width: 5.5em;
  height: 5.5em;
  border-radius: 1em;
  background: var(--c);
  color: #fff;
  font-family: system-ui, sans-serif;
  font-weight: 700;
  box-shadow: 0 0.6em 1.4em -0.6em color-mix(in srgb, var(--c) 70%, transparent);
  border-radius: 0.6em;
  animation: kfa-square-to-circle-morph-kf 3.2s cubic-bezier(0.65, 0, 0.35, 1) infinite;
}
@keyframes kfa-square-to-circle-morph-kf {
  0%, 8% { border-radius: 0.6em; transform: rotate(0); }
  42%, 58% { border-radius: 50%; transform: rotate(180deg) scale(0.9); }
  92%, 100% { border-radius: 0.6em; transform: rotate(360deg); }
}

@media (prefers-reduced-motion: reduce) {
  .kfa-square-to-circle-morph, .kfa-square-to-circle-morph *, .kfa-square-to-circle-morph::before, .kfa-square-to-circle-morph::after, .kfa-square-to-circle-morph *::before, .kfa-square-to-circle-morph *::after {
    animation-duration: 0.01ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.01ms !important;
  }
}

Customizing this snippet

Color: set --kfa-color on the element or a parent. Currently previewing #3b82f6.
Base size: set --kfa-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.

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