<h1 class="title">アコーディオン実装例</h1>
<div class="accordion">
  <h2 class="accordion-header">
    <button class="accordion-trigger" aria-expanded="false" aria-controls="accordion-panel-1">
      見出し01<span class="accordion-icon"></span>
    </button>
  </h2>
  <div id="accordion-panel-1" class="accordion-panel __close">
    <p class="accordion-panel__text">
      テキストが入ります。テキストが入ります。テキストが入ります。
    </p>
  </div>
  <h2 class="accordion-header">
    <button class="accordion-trigger" aria-expanded="false" aria-controls="accordion-panel-2">
      見出し02<span class="accordion-icon"></span>
    </button>
  </h2>
  <div id="accordion-panel-2" class="accordion-panel __close">
    <p class="accordion-panel__text">
      テキストが入ります。テキストが入ります。テキストが入ります。
    </p>
  </div>
  <h2 class="accordion-header">
    <button class="accordion-trigger" aria-expanded="false" aria-controls="accordion-panel-3">
      見出し03<span class="accordion-icon"></span>
    </button>
  </h2>
  <div id="accordion-panel-3" class="accordion-panel __close">
    <p class="accordion-panel__text">
      テキストが入ります。テキストが入ります。テキストが入ります。
    </p>
  </div>
</div>

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}
body {
  padding: 40px 0;
  background-color: #efefef;
  font-family: sans-serif;
  line-height: 1.5;
}
button {
  border-style: none;
  color: inherit;
  font-family: sans-serif;
}
.title {
  font-size: 1.25rem;
  font-weight: 600;
  text-align: center;
}
.accordion {
  width: 400px;
  max-width: 90%;
  margin-top: 2rem;
  margin-left: auto;
  margin-right: auto;
  border: 2px solid #a3a3a3;
}
.accordion > * + * {
  border-top: 1px solid #a3a3a3;
}
.accordion-header {
  position: relative;
}
.accordion-trigger {
  display: block;
  width: 100%;
  padding: 1rem 3rem 1rem 1.5rem;
  font-size: 1.25rem;
  text-align: left;
  font-weight: bold;
  cursor: pointer;
}
.accordion-trigger[aria-expanded="true"] {
  background-color: #dbeafe;
}
@media (hover: hover) {
  .accordion-trigger:hover {
    background-color: #dbeafe;
  }
}
.accordion-icon {
  position: absolute;
  top: 0;
  right: 1rem;
  bottom: 0;
  width: 10px;
  height: 10px;
  margin: auto;
  border: solid #000;
  border-width: 2px 2px 0 0;
  transform: rotate(135deg);
}
.accordion-trigger[aria-expanded="true"] .accordion-icon {
  transform: rotate(-45deg);
}
.accordion-panel.__close {
  display: none;
}
.accordion-panel__text {
  padding: 1rem 1.5rem;
}

const triggers = document.querySelectorAll(".accordion-trigger");
triggers.forEach((trigger) => {
  const controls = trigger.getAttribute("aria-controls");
  const panel = document.getElementById(controls);

  trigger.addEventListener("click", (e) => {
    const target = e.currentTarget;
    const isOpen = target.getAttribute("aria-expanded") === "true";

    if (isOpen) {
      // アコーディオンを閉じる
      target.setAttribute("aria-expanded", "false");
      panel.classList.add("__close");
    } else {
      // アコーディオンを開く
      target.setAttribute("aria-expanded", "true");
      panel.classList.remove("__close");
    }
  });
});