Snake Border
A line with a fading tail runs around the box like a snake and turns every corner. Four edge spans share one clock, each offset by its distance around the box, so the head never jumps.
Also known as: snake border animation css, running border animation, border trace animation.
Syncing files
HTML
<div class="bdr-snake-border"> <span class="bdr-snake-border-top"></span> <span class="bdr-snake-border-right"></span> <span class="bdr-snake-border-bottom"></span> <span class="bdr-snake-border-left"></span> Syncing files </div>
CSS
.bdr-snake-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;
--t: 3.6s;
--w: 0.2em;
--tail: 5.5em;
overflow: hidden;
border-radius: 0.3em;
background: color-mix(in srgb, var(--c) 6%, transparent);
box-shadow: inset 0 0 0 1px color-mix(in srgb, var(--c) 18%, transparent);
}
.bdr-snake-border-top,
.bdr-snake-border-right,
.bdr-snake-border-bottom,
.bdr-snake-border-left {
position: absolute;
background-repeat: no-repeat;
filter: drop-shadow(0 0 0.2em var(--c));
animation: bdr-snake-border-x var(--t) linear infinite;
}
.bdr-snake-border-top,
.bdr-snake-border-bottom {
left: 0;
width: 100%;
height: var(--w);
background-image: linear-gradient(90deg, transparent, var(--c) 75%);
background-size: var(--tail) 100%;
}
.bdr-snake-border-right,
.bdr-snake-border-left {
top: 0;
width: var(--w);
height: 100%;
background-image: linear-gradient(180deg, transparent, var(--c) 75%);
background-size: 100% var(--tail);
animation-name: bdr-snake-border-y;
}
.bdr-snake-border-top { top: 0; }
.bdr-snake-border-right { right: 0; animation-delay: calc(var(--t) * -0.681); }
.bdr-snake-border-bottom { bottom: 0; rotate: 180deg; animation-delay: calc(var(--t) * -0.5); }
.bdr-snake-border-left { left: 0; rotate: 180deg; animation-delay: calc(var(--t) * -0.181); }
/* The box is 15em by 8.5em, 47em around: an edge takes its share of the loop. */
@keyframes bdr-snake-border-x {
0% { transform: translateX(calc(var(--tail) * -1)); }
31.9% { transform: translateX(calc(100% - var(--tail))); }
42.55%, 100% { transform: translateX(100%); }
}
@keyframes bdr-snake-border-y {
0% { transform: translateY(calc(var(--tail) * -1)); }
18.09% { transform: translateY(calc(100% - var(--tail))); }
28.72%, 100% { transform: translateY(100%); }
}
@media (prefers-reduced-motion: reduce) {
.bdr-snake-border, .bdr-snake-border *, .bdr-snake-border::before, .bdr-snake-border::after, .bdr-snake-border *::before, .bdr-snake-border *::after {
animation-duration: 0.01ms !important;
animation-iteration-count: 1 !important;
transition-duration: 0.01ms !important;
}
}Customizing this snippet
- Color: set
--bdr-coloron the element or a parent. Currently previewing#3b82f6. - Base size: set
--bdr-size. Every other dimension is inem, so the whole thing scales together. Currently previewing16px. - Speed: change the duration on the
transitionoranimationshorthand. 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.