CSSのみでアコーディオンメニューを作成する方法｜:target疑似クラスとcheckboxを活用

.accordion {
  max-width: 600px;
  margin: 20px auto;
  border: 1px solid #ddd;
  border-radius: 8px;
  overflow: hidden;
}

.accordion-item {
  border-bottom: 1px solid #eee;
}

.accordion-item:last-child {
  border-bottom: none;
}

.accordion-trigger {
  display: block;
  padding: 16px 20px;
  background-color: #f8f9fa;
  color: #333;
  text-decoration: none;
  border: none;
  cursor: pointer;
  font-size: 16px;
  position: relative;
  transition: background-color 0.2s;
}

.accordion-trigger:hover {
  background-color: #e9ecef;
}

.accordion-trigger::after {
  content: '+';
  position: absolute;
  right: 20px;
  top: 50%;
  transform: translateY(-50%);
  font-size: 20px;
  line-height: 1;
  transition: transform 0.2s;
}

.accordion-content {
  max-height: 0;
  overflow: hidden;
  transition: max-height 0.3s ease;
  background-color: #fff;
}

.accordion-content p {
  padding: 16px 20px;
  margin: 0;
  line-height: 1.6;
}

/* :target疑似クラスでアコーディオンを開く */
.accordion-content:target {
  max-height: 200px;
}

.accordion-content:target + .accordion-trigger::after {
  transform: translateY(-50%) rotate(45deg);
}

.checkbox-accordion {
  max-width: 600px;
  margin: 20px auto;
  border: 1px solid #ddd;
  border-radius: 8px;
  overflow: hidden;
}

.accordion-item {
  border-bottom: 1px solid #eee;
}

.accordion-item:last-child {
  border-bottom: none;
}

.accordion-checkbox {
  display: none;
}

.accordion-label {
  display: block;
  padding: 16px 20px;
  background-color: #f8f9fa;
  color: #333;
  cursor: pointer;
  font-size: 16px;
  position: relative;
  transition: background-color 0.2s;
  user-select: none;
}

.accordion-label:hover {
  background-color: #e9ecef;
}

.accordion-label::after {
  content: '+';
  position: absolute;
  right: 20px;
  top: 50%;
  transform: translateY(-50%);
  font-size: 20px;
  line-height: 1;
  transition: transform 0.3s ease;
}

.accordion-panel {
  max-height: 0;
  overflow: hidden;
  transition: max-height 0.4s ease;
  background-color: #fff;
}

.accordion-inner {
  padding: 16px 20px;
}

.accordion-inner p {
  margin: 0;
  line-height: 1.6;
}

/* チェックされた時の表示 */
.accordion-checkbox:checked + .accordion-label + .accordion-panel {
  max-height: 200px;
}

.accordion-checkbox:checked + .accordion-label::after {
  transform: translateY(-50%) rotate(45deg);
}

.accordion-checkbox:checked + .accordion-label {
  background-color: #e3f2fd;
}

.enhanced-checkbox-accordion {
  max-width: 600px;
  margin: 20px auto;
  background: white;
  border-radius: 10px;
  box-shadow: 0 2px 10px rgba(0,0,0,0.1);
  overflow: hidden;
}

.enhanced-checkbox-accordion .accordion-item {
  border-bottom: 1px solid #f0f0f0;
}

.enhanced-checkbox-accordion .accordion-item:last-child {
  border-bottom: none;
}

.enhanced-checkbox-accordion .accordion-toggle {
  display: none;
}

.enhanced-checkbox-accordion .accordion-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 18px 24px;
  background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
  color: white;
  cursor: pointer;
  user-select: none;
  transition: all 0.3s ease;
}

.enhanced-checkbox-accordion .accordion-header:hover {
  background: linear-gradient(135deg, #5a6fd8 0%, #6a4190 100%);
  transform: translateY(-1px);
}

.enhanced-checkbox-accordion .accordion-title {
  font-weight: 500;
  font-size: 16px;
}

.enhanced-checkbox-accordion .accordion-icon {
  width: 12px;
  height: 12px;
  position: relative;
  transition: transform 0.3s ease;
}

.enhanced-checkbox-accordion .accordion-icon::before,
.enhanced-checkbox-accordion .accordion-icon::after {
  content: '';
  position: absolute;
  background-color: white;
  transition: transform 0.3s ease;
}

.enhanced-checkbox-accordion .accordion-icon::before {
  width: 12px;
  height: 2px;
  top: 5px;
  left: 0;
}

.enhanced-checkbox-accordion .accordion-icon::after {
  width: 2px;
  height: 12px;
  top: 0;
  left: 5px;
}

.enhanced-checkbox-accordion .accordion-content {
  max-height: 0;
  overflow: hidden;
  transition: max-height 0.4s cubic-bezier(0.25, 0.46, 0.45, 0.94);
  background: #f8f9fc;
}

.enhanced-checkbox-accordion .accordion-body {
  padding: 20px 24px;
  border-top: 1px solid #e1e5f2;
}

.enhanced-checkbox-accordion .accordion-body p {
  margin: 0 0 12px 0;
  line-height: 1.6;
  color: #555;
}

.enhanced-checkbox-accordion .accordion-body p:last-child {
  margin-bottom: 0;
}

/* チェック時のアニメーション */
.enhanced-checkbox-accordion .accordion-toggle:checked + .accordion-header + .accordion-content {
  max-height: 300px;
}

.enhanced-checkbox-accordion .accordion-toggle:checked + .accordion-header .accordion-icon::after {
  transform: rotate(90deg);
}

.enhanced-checkbox-accordion .accordion-toggle:checked + .accordion-header {
  background: linear-gradient(135deg, #4facfe 0%, #00f2fe 100%);
}