アコーディオン：シンプルプラス｜Accordion: Simple Plus

<div class="container">
  <div class="btn-box">
      <button data-default-text="Borderless Simple Accordion" data-open-text="Close">Borderless Simple Accordion</button>
  </div>
  <div class="more">
      <ul>
          <li>Clean borderless design</li>
          <li>Simple plus/minus icon</li>
          <li>Minimal and elegant</li>
          <li>Perfect for clean layouts</li>
          <li>No visual clutter</li>
      </ul>
  </div>
</div>

@keyframes simpleFadeIn {
  0% {
    opacity: 0;
    transform: translateY(-5px);
  }
  100% {
    opacity: 1;
    transform: translateY(0);
  }
}

.container {
  margin: 20px 0;
  position: relative;
  max-width: 600px;
  margin-left: auto;
  margin-right: auto;
}

.container .btn-box button {
  width: 100%;
  padding: 15px 0;
  background: transparent;
  border: none;
  font-size: 16px;
  font-weight: 400;
  color: #333;
  cursor: pointer;
  transition: all 0.3s ease;
  text-align: left;
  position: relative;
  font-family: 'Arial', sans-serif;
}

.container .btn-box button::after {
  content: '+';
  position: absolute;
  right: 0;
  top: 50%;
  transform: translateY(-50%);
  font-size: 20px;
  font-weight: 300;
  color: #666;
  transition: all 0.3s ease;
  width: 20px;
  height: 20px;
  display: flex;
  align-items: center;
  justify-content: center;
}

.container .btn-box button:hover {
  color: #000;
}

.container .btn-box button:hover::after {
  color: #000;
}

.container .more {
  max-height: 0;
  overflow: hidden;
  transition: max-height 0.4s ease;
  background: transparent;
}

.container .more.appear {
  max-height: 400px;
  animation: simpleFadeIn 0.3s ease;
}

.container:has(.more.appear) .btn-box button::after {
  content: '−';
  transform: translateY(-50%);
  color: #000;
}

.container .more ul {
  list-style: none;
  padding: 15px 0;
  margin: 0;
}

.container .more li {
  padding: 8px 0;
  color: #666;
  font-size: 14px;
  line-height: 1.6;
}

/* レスポンシブ対応 */
@media (max-width: 768px) {
  .container .btn-box button {
    font-size: 14px;
    padding: 12px 0;
  }
  
  .container .more ul {
    padding: 10px 0;
  }
  
  .container .more.appear {
    max-height: 300px;
  }
}

document.addEventListener('DOMContentLoaded', function() {
    const button = document.querySelector('.btn-box button');
    const content = document.querySelector('.more');
    
    button.addEventListener('click', function() {
        content.classList.toggle('appear');
        
        if (content.classList.contains('appear')) {
            this.textContent = this.dataset.openText || 'Close';
        } else {
            this.textContent = this.dataset.defaultText;
        }
    });
});