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{} Modern CSS & web APIs

@starting-style

An at-rule that defines the "before" state for an element appearing from `display:none`, so entry animations work with plain transitions.

Also known as: Entry transitionDisplay transition
···
html
<div class="wrap">
  <div class="badge" id="badge"><svg viewBox="0 0 10 10"><path id="bpath" d="M1.5 5.2l2.6 2.6L8.5 2.4" fill="none" stroke="currentColor" stroke-width="1.7" stroke-linecap="round" stroke-linejoin="round"/></svg><span id="btext">확인 중</span></div>
  <button class="chip" id="card">등장 카드</button>
</div>
css
.wrap{position:relative;width:100%;height:100%;display:grid;place-items:center}
.chip{width:180px;height:70px;border-radius:12px;border:1px solid var(--line);background:var(--surface);color:var(--fg);
  font-size:13px;font-weight:600;opacity:1;scale:1;transition:opacity .5s,scale .5s;transition-behavior:allow-discrete}
@starting-style{ .chip.enter{ opacity:0; scale:.85 } }
.chip.fallback-anim{animation:pop 2.6s ease infinite}
@keyframes pop{0%{opacity:0;scale:.85}20%,80%{opacity:1;scale:1}100%{opacity:0;scale:.85}}
.badge{position:absolute;top:10px;right:10px;display:flex;align-items:center;gap:5px;padding:4px 9px;border-radius:999px;font-size:10px;font-weight:700;background:var(--bg);border:1px solid var(--line);color:var(--accent-3);z-index:2}
.badge.no{color:var(--accent-2)}
.badge svg{width:9px;height:9px}
js
const ok = CSS.supports('transition-behavior', 'allow-discrete');
const b=document.getElementById('badge'),p=document.getElementById('bpath'),t=document.getElementById('btext');
b.classList.toggle('no', !ok);
p.setAttribute('d', ok ? 'M1.5 5.2l2.6 2.6L8.5 2.4' : 'M2 2l6 6M8 2l-6 6');
t.textContent = ok ? '@starting-style 지원됨' : '미지원 · keyframes 폴백';
const card = document.getElementById('card');
if (ok) {
  let shown = true;
  setInterval(() => {
    shown = !shown;
    if (shown) { card.style.display = 'block'; requestAnimationFrame(() => card.classList.remove('enter')); }
    else { card.classList.add('enter'); setTimeout(() => card.style.display = 'none', 500); }
  }, 2000);
} else {
  card.classList.add('fallback-anim');
}

CSS transitions were originally limited to property changes on elements already on screen. An element going from `display:none` to `display:block` has no "before" value to transition from, so fade-in modals needed keyframe animations or a JS trick — add a class one `requestAnimationFrame` late.

Values defined inside `@starting-style` are exactly that "just before it appears" frame. Combined with `transition-behavior: allow-discrete`, even discrete properties like `display` and `overlay` can transition, so a dialog or popover fading in from `display:none` becomes a one-line transition.

Check caniuse/MDN baseline for support. Where it's missing, a `@keyframes` entry animation remains the standard fallback.

When to use

Giving a modal, popover, or toast a smooth fade-in from display:none without writing keyframes.