05색이 흐르는 밑줄

<main class="stage">
  <a class="link is-demo" href="#">지금 가입하기</a>
</main>

@use "../../../../site/static/demo-tokens" as *;

// ── 변수 3개 ─────────────────────────────
$duration: $dur-quick;
$easing:   $ease-out;
$color:    $subject-blue;

// ── 데모 무대 ───────────────────────────────
@include stage($stage-ink);

.link {
  position: relative;
  display: inline-block;
  padding: $sp-2 $sp-1;
  color: $subject-cream;
  text-decoration: none;
  font-weight: 700; font-size: clamp(18px, 5vw, 26px); line-height: 1.2;

  &::after {
    content: "";
    position: absolute;
    left: 0; right: 0; bottom: 0;
    height: 3px;
    background: linear-gradient(90deg, $color, $stage-yellow, $color);
    background-size: 200% 100%;
    background-position: 0% 0;
    transform: scaleX(0);
    transform-origin: left center; // 규칙8: 언더라인 좌측 채움
    transition: transform $duration $easing;
  }
  // 실제 브라우저 — scaleX 는 트랜지션, 색 흐름은 호버 중에만 도는 애니메이션
  &:hover::after, &:focus-visible::after {
    transform: scaleX(1);
    animation: color-flow 1.4s linear infinite;
  }

  // 렌더러는 :hover 를 못 흉내 내므로 하나의 키프레임에 그리기·흐름·지우기를 전부 담는다
  &.is-demo::after {
    animation: color-slide-loop $dur-loop $easing infinite;
  }
}

@keyframes color-flow {
  0%   { background-position: 0% 0; }
  100% { background-position: 100% 0; }
}

@keyframes color-slide-loop {
  0%   { transform: scaleX(0); background-position: 0% 0; }
  15%  { transform: scaleX(1); background-position: 0% 0; }
  85%  { transform: scaleX(1); background-position: 100% 0; }
  100% { transform: scaleX(0); background-position: 100% 0; }
}

@media (prefers-reduced-motion: reduce) {
  .link::after, .link.is-demo::after { transition: none; animation: none; }
  .link:hover::after, .link:focus-visible::after { transform: scaleX(1); }
}

html, body {
  margin: 0;
  overflow: hidden;
}

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #17141a;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.link {
  position: relative;
  display: inline-block;
  padding: 8px 4px;
  color: #fff7e6;
  text-decoration: none;
  font-weight: 700;
  font-size: clamp(18px, 5vw, 26px);
  line-height: 1.2;
}
.link::after {
  content: "";
  position: absolute;
  left: 0;
  right: 0;
  bottom: 0;
  height: 3px;
  background: linear-gradient(90deg, #2f6df6, #ffd23f, #2f6df6);
  background-size: 200% 100%;
  background-position: 0% 0;
  transform: scaleX(0);
  transform-origin: left center;
  transition: transform 220ms ease-out;
}
.link:hover::after, .link:focus-visible::after {
  transform: scaleX(1);
  animation: color-flow 1.4s linear infinite;
}
.link.is-demo::after {
  animation: color-slide-loop 2s ease-out infinite;
}

@keyframes color-flow {
  0% {
    background-position: 0% 0;
  }
  100% {
    background-position: 100% 0;
  }
}
@keyframes color-slide-loop {
  0% {
    transform: scaleX(0);
    background-position: 0% 0;
  }
  15% {
    transform: scaleX(1);
    background-position: 0% 0;
  }
  85% {
    transform: scaleX(1);
    background-position: 100% 0;
  }
  100% {
    transform: scaleX(0);
    background-position: 100% 0;
  }
}
@media (prefers-reduced-motion: reduce) {
  .link::after, .link.is-demo::after {
    transition: none;
    animation: none;
  }
  .link:hover::after, .link:focus-visible::after {
    transform: scaleX(1);
  }
}

document.querySelectorAll('.link').forEach(function (b) {
  b.addEventListener('pointerenter', function () { b.classList.remove('is-demo'); });
  b.addEventListener('click', function (e) { e.preventDefault(); });
});

import type { CSSProperties, ReactNode } from "react";
import styles from "./ColorSlideUnderline.module.scss";

export type ColorSlideUnderlineProps = { children: ReactNode; href: string; duration?: number; color?: string; accent?: string };

export default function ColorSlideUnderline({ children, href, duration = 220, color = "#2f6df6", accent = "#ffd23f" }: ColorSlideUnderlineProps) {
  const vars = { "--duration": `${duration}ms`, "--color": color, "--accent": accent } as CSSProperties;
  return (
    <a className={styles.link} href={href} style={vars}>
      {children}
    </a>
  );
}