Gradient Bottom Border
Only the bottom edge carries the border: a gradient bar that flows left to right under a panel, with a soft glow beneath. One background layer sized to the bottom strip loops its position.
Also known as: gradient border bottom css, animated border bottom css, gradient underline border.
Section heading
HTML
<div class="bdr-gradient-bottom-border">Section heading</div>
CSS
.bdr-gradient-bottom-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;
--bar: linear-gradient(90deg, var(--c), color-mix(in oklch, var(--c), #ec4899 80%), color-mix(in oklch, var(--c), #f59e0b 75%), var(--c));
border-radius: 0.8em 0.8em 0.25em 0.25em;
background: var(--bar) 0 100% / 15em 0.22em repeat-x, color-mix(in srgb, var(--c) 8%, transparent);
animation: bdr-gradient-bottom-border-flow 3s linear infinite;
}
.bdr-gradient-bottom-border::after {
content: "";
position: absolute;
inset: auto 1em -0.25em;
height: 0.35em;
background: var(--bar) 0 0 / 15em 100% repeat-x;
filter: blur(0.45em);
opacity: 0.7;
pointer-events: none;
animation: bdr-gradient-bottom-border-flow 3s linear infinite;
}
@keyframes bdr-gradient-bottom-border-flow {
from { background-position: -1em 100%, 0 0; }
to { background-position: 14em 100%, 0 0; }
}
@media (prefers-reduced-motion: reduce) {
.bdr-gradient-bottom-border, .bdr-gradient-bottom-border *, .bdr-gradient-bottom-border::before, .bdr-gradient-bottom-border::after, .bdr-gradient-bottom-border *::before, .bdr-gradient-bottom-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.