09カウントダウンタイマー

<main class="stage">
  <div class="counter is-demo">
    <span class="counter__num" aria-hidden="true"></span>
    <span class="counter__sr">0</span>
    <span class="counter__bar" aria-hidden="true"></span>
    <span class="counter__label">終了まで</span>
  </div>
</main>

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

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

@include stage($stage-ink);

@property --t {
  syntax: '<integer>';
  initial-value: 9;
  inherits: true;                                     // 숫자와 아래 바 둘 다 같은 값을 읽는다
}

.counter {
  --t: 9;
  text-align: center;
  color: $color;                                       // 남은 수가 줄어들수록 이 색이 경고색으로 바뀐다

  &__num {
    counter-reset: t var(--t);
    display: block;
    font-weight: 800; font-size: clamp(44px, 14vw, 72px); line-height: 1; font-family: inherit;
    font-variant-numeric: tabular-nums;
    color: inherit;

    &::before { content: counter(t); }
  }

  &__sr {
    position: absolute;
    width: 1px; height: 1px;
    overflow: hidden;
    clip-path: inset(50%);
    white-space: nowrap;
  }

  // 남은 수만큼 채워진 바 — 숫자가 줄어드는 만큼 짧아져 변한 픽셀 면적을 키운다
  &__bar {
    display: block;
    width: clamp(90px, 26vw, 130px);
    height: 6px;
    margin: clamp(10px, 3vw, 14px) auto 0;
    border-radius: $r-pill;
    background: rgba(255, 255, 255, .18);
    overflow: hidden;

    &::after {
      content: "";
      display: block;
      height: 100%;
      background: currentColor;                        // 부모(.counter)의 color 를 그대로 따라간다
      transform-origin: left; // 규칙8: 남은 시간 왼쪽 기준 바
      transform: scaleX(calc(var(--t) / 9));
    }
  }

  &__label {
    display: block;
    margin-top: clamp(6px, 2vw, 10px);
    font-weight: 600; font-size: clamp(12px, 3.4vw, 15px); line-height: 1; font-family: inherit;
    color: rgba(255, 255, 255, .6);
  }
}

// 목표값이 아니라 0을 향해 거꾸로 센다. 색은 부모에서 함께 애니메이션해 숫자·바가 같이 바뀐다
.counter.is-demo {
  animation: count-down $duration $easing infinite;
}

@keyframes count-down {
  0%   { --t: 9; color: $color; }
  60%  { --t: 3; color: $color; }
  85%  { --t: 0; color: $error; }
  100% { --t: 0; color: $error; }
}

@media (prefers-reduced-motion: reduce) {
  .counter.is-demo { animation: none; --t: 0; color: $error; }
}

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);
}

@property --t {
  syntax: "<integer>";
  initial-value: 9;
  inherits: true;
}
.counter {
  --t: 9;
  text-align: center;
  color: #fff7e6;
}
.counter__num {
  counter-reset: t var(--t);
  display: block;
  font-weight: 800;
  font-size: clamp(44px, 14vw, 72px);
  line-height: 1;
  font-family: inherit;
  font-variant-numeric: tabular-nums;
  color: inherit;
}
.counter__num::before {
  content: counter(t);
}
.counter__sr {
  position: absolute;
  width: 1px;
  height: 1px;
  overflow: hidden;
  clip-path: inset(50%);
  white-space: nowrap;
}
.counter__bar {
  display: block;
  width: clamp(90px, 26vw, 130px);
  height: 6px;
  margin: clamp(10px, 3vw, 14px) auto 0;
  border-radius: 999px;
  background: rgba(255, 255, 255, 0.18);
  overflow: hidden;
}
.counter__bar::after {
  content: "";
  display: block;
  height: 100%;
  background: currentColor;
  transform-origin: left;
  transform: scaleX(calc(var(--t) / 9));
}
.counter__label {
  display: block;
  margin-top: clamp(6px, 2vw, 10px);
  font-weight: 600;
  font-size: clamp(12px, 3.4vw, 15px);
  line-height: 1;
  font-family: inherit;
  color: rgba(255, 255, 255, 0.6);
}

.counter.is-demo {
  animation: count-down 2s ease-out infinite;
}

@keyframes count-down {
  0% {
    --t: 9;
    color: #fff7e6;
  }
  60% {
    --t: 3;
    color: #fff7e6;
  }
  85% {
    --t: 0;
    color: #c2361a;
  }
  100% {
    --t: 0;
    color: #c2361a;
  }
}
@media (prefers-reduced-motion: reduce) {
  .counter.is-demo {
    animation: none;
    --t: 0;
    color: #c2361a;
  }
}

import type { CSSProperties } from "react";
import styles from "./CountdownTimer.module.scss";

export type CountdownTimerProps = {
  /** 몇에서 시작해 0으로 향할지 */
  from: number;
  label: string;
  duration?: number;
  easing?: string;
  color?: string;
};

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string; "--color"?: string; "--t-start"?: number };

export default function CountdownTimer({
  from, label, duration = 2000, easing = "ease-out", color = "#fff7e6",
}: CountdownTimerProps) {
  const vars: Vars = {
    "--duration": `${duration}ms`,
    "--easing": easing,
    "--color": color,
    "--t-start": from,
  };

  return (
    <div className={styles.counter} style={vars}>
      <span className={styles.num} aria-hidden="true" />
      <span className={styles.sr}>0</span>
      <span className={styles.bar} aria-hidden="true" />
      <span className={styles.label}>{label}</span>
    </div>
  );
}