animation-fill-mode

animation-fill-mode: none;
animation-delay: 1s;

animation-fill-mode: forwards;
animation-delay: 1s;

animation-fill-mode: backwards;
animation-delay: 1s;

animation-fill-mode: both;
animation-delay: 1s;

<section class="flex-column" id="default-example">
  <div>アニメーション<span id="playstatus"></span></div>
  <div id="example-element">モードを選択すると開始します！</div>
</section>

#example-element {
  background-color: #1766aa;
  color: white;
  margin: auto;
  margin-left: 0;
  border: 5px solid #333;
  width: 150px;
  height: 150px;
  border-radius: 50%;

  display: flex;
  justify-content: center;
  align-items: center;
  flex-direction: column;
}

#playstatus {
  font-weight: bold;
}

.animating {
  animation: slide 1s ease-in 1;
}

@keyframes slide {
  from {
    background-color: orange;
    color: black;
    margin-left: 0;
  }
  to {
    background-color: orange;
    color: black;
    margin-left: 80%;
  }
}

"use strict";

window.addEventListener("load", () => {
  const el = document.getElementById("example-element");
  const status = document.getElementById("playstatus");

  function update() {
    status.textContent = "待機中";
    el.className = "";
    window.requestAnimationFrame(() => {
      window.requestAnimationFrame(() => {
        el.className = "animating";
      });
    });
  }

  el.addEventListener("animationstart", () => {
    status.textContent = "動作中";
  });

  el.addEventListener("animationend", () => {
    status.textContent = "完了";
  });

  const observer = new MutationObserver(() => {
    update();
  });

  observer.observe(el, {
    attributes: true,
    attributeFilter: ["style"],
  });

  update();
});

/* 単一のアニメーション */
animation-fill-mode: none;
animation-fill-mode: forwards;
animation-fill-mode: backwards;
animation-fill-mode: both;

/* 複数のアニメーション */
animation-fill-mode: none, backwards;
animation-fill-mode: both, forwards, none;

/* グローバル値 */
animation-fill-mode: inherit;
animation-fill-mode: initial;
animation-fill-mode: revert;
animation-fill-mode: revert-layer;
animation-fill-mode: unset;

<p>マウスを灰色のボックスの上に乗せてください！</p>
<div class="demo">
  <div class="grows-and-stays">これは大きくなって大きいままになります。</div>
  <div class="grows">これは大きくなるだけです。</div>
</div>

.demo {
  border-top: 100px solid #ccc;
  height: 300px;
}

@keyframes grow {
  0% {
    font-size: 0;
  }
  100% {
    font-size: 40px;
  }
}

.demo:hover .grows {
  animation-name: grow;
  animation-duration: 3s;
}

.demo:hover .grows-and-stays {
  animation-name: grow;
  animation-duration: 3s;
  animation-fill-mode: forwards;
}