01Fade lúc mount bằng @starting-style

<main class="stage">
  <div class="mf-wrap is-demo">
    <div class="mf-card">
      <span class="mf-card__dot"></span>
      <span class="mf-card__title">New card</span>
      <span class="mf-card__sub">mounted just now</span>
    </div>
  </div>
</main>

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

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

@include stage($stage-ink);

.mf-wrap {
  width: min(320px, 90vw);
  display: grid;
  place-items: center;
}

.mf-card {
  width: min(260px, 84vw);
  box-sizing: border-box;
  padding: $sp-5 $sp-5;
  border-radius: $r-card;
  background: linear-gradient(160deg, $color, rgba(23, 20, 26, .9));
  box-shadow: $shadow-raised;
  display: flex;
  flex-direction: column;
  gap: $sp-1;
  color: $subject-cream;
  font-family: -apple-system, "Segoe UI", sans-serif;
}

.mf-card__dot {
  width: $sp-3;
  height: $sp-3;
  border-radius: $r-pill;
  background: $subject-cream;
  margin-bottom: $sp-2;
}

.mf-card__title { font-weight: 700; font-size: 16px; line-height: 1.2; font-family: inherit; }
.mf-card__sub { font-weight: 400; font-size: 12px; line-height: 1.4; font-family: inherit; opacity: .72; }

// 실사용 원리: React 19 는 새 노드가 DOM 에 붙는 순간 @starting-style 의 값에서
// 실제 스타일로 트랜지션한다 (useEffect 로 "mounted" 플래그를 따로 안 만들어도 됨).
// 여기 데모는 그 등장 곡선을 무한 반복으로 보여준다.
@starting-style {
  .mf-card { opacity: 0; transform: translateY(10px) scale(.94); }
}
.mf-card {
  opacity: 1;
  transform: none;
  transition: opacity $duration $easing, transform $duration $easing;
}

// 캡처용 루프: 마운트 순간을 2초마다 재생 (demo.py 는 WAAPI 로 이 keyframes 를 직접 스크럽한다)
@keyframes mf-mount {
  0%   { opacity: 0; transform: translateY(10px) scale(.94); }
  22%  { opacity: 1; transform: none; }
  82%  { opacity: 1; transform: none; }
  100% { opacity: 0; transform: translateY(10px) scale(.94); }
}
.mf-wrap.is-demo .mf-card { animation: mf-mount $dur-loop $easing infinite; }

@media (prefers-reduced-motion: reduce) {
  .mf-card { transition: none; }
  .mf-wrap.is-demo .mf-card { animation: none !important; opacity: 1; transform: none; }
}

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);
}

.mf-wrap {
  width: min(320px, 90vw);
  display: grid;
  place-items: center;
}

.mf-card {
  width: min(260px, 84vw);
  box-sizing: border-box;
  padding: 20px 20px;
  border-radius: 16px;
  background: linear-gradient(160deg, #2f6df6, rgba(23, 20, 26, 0.9));
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
  display: flex;
  flex-direction: column;
  gap: 4px;
  color: #fff7e6;
  font-family: -apple-system, "Segoe UI", sans-serif;
}

.mf-card__dot {
  width: 12px;
  height: 12px;
  border-radius: 999px;
  background: #fff7e6;
  margin-bottom: 8px;
}

.mf-card__title {
  font-weight: 700;
  font-size: 16px;
  line-height: 1.2;
  font-family: inherit;
}

.mf-card__sub {
  font-weight: 400;
  font-size: 12px;
  line-height: 1.4;
  font-family: inherit;
  opacity: 0.72;
}

@starting-style {
  .mf-card {
    opacity: 0;
    transform: translateY(10px) scale(0.94);
  }
}
.mf-card {
  opacity: 1;
  transform: none;
  transition: opacity 600ms cubic-bezier(0.2, 0.8, 0.2, 1), transform 600ms cubic-bezier(0.2, 0.8, 0.2, 1);
}

@keyframes mf-mount {
  0% {
    opacity: 0;
    transform: translateY(10px) scale(0.94);
  }
  22% {
    opacity: 1;
    transform: none;
  }
  82% {
    opacity: 1;
    transform: none;
  }
  100% {
    opacity: 0;
    transform: translateY(10px) scale(0.94);
  }
}
.mf-wrap.is-demo .mf-card {
  animation: mf-mount 2s cubic-bezier(0.2, 0.8, 0.2, 1) infinite;
}

@media (prefers-reduced-motion: reduce) {
  .mf-card {
    transition: none;
  }
  .mf-wrap.is-demo .mf-card {
    animation: none !important;
    opacity: 1;
    transform: none;
  }
}

import { CSSProperties, useEffect, useState } from "react";
import styles from "./MountFadeStartingStyle.module.scss";

export interface MountFadeStartingStyleProps {
  duration?: string;
  easing?: string;
  color?: string;
}

function Card({ color }: { color: string }) {
  // React 19: @starting-style (in the CSS module below) defines what this card looked
  // like before it existed in the DOM, so no useEffect("mounted") flag is needed for
  // the fade-in — the browser transitions from that starting state automatically.
  return (
    <div className={styles.card} style={{ background: color }}>
      <span className={styles.dot} />
      <span className={styles.title}>New card</span>
      <span className={styles.sub}>mounted just now</span>
    </div>
  );
}

export default function MountFadeStartingStyle({
  duration = "600ms",
  easing = "cubic-bezier(.2, .8, .2, 1)",
  color = "#2f6df6",
}: MountFadeStartingStyleProps) {
  const [mounted, setMounted] = useState(true);

  useEffect(() => {
    const id = setInterval(() => setMounted((m) => !m), 1400);
    return () => clearInterval(id);
  }, []);

  const style: CSSProperties & { "--duration"?: string; "--easing"?: string } = {
    "--duration": duration,
    "--easing": easing,
  };

  return (
    <div className={styles.wrap} style={style}>
      {mounted ? <Card color={color} /> : null}
    </div>
  );
}