【脱・JavaScript】CSSだけで作る！リッチに動くハンバーガーメニュー完全ガイド

<div class="menu-wrapper">
  <input type="checkbox" id="menu-toggle" hidden>
  
  <label class="menu-icon" for="menu-toggle">
    <span></span>
    <span></span>
    <span></span>
  </label>
  
  <div class="overlay"></div>
  
  <nav class="menu">
    <ul>
      <li><a href="#">Home</a></li>
      <li><a href="#">About</a></li>
      <li><a href="#">Service</a></li>
      <li><a href="#">Contact</a></li>
    </ul>
  </nav>
</div>

.menu-wrapper {
  position: relative;
}

.menu-icon {
  width: 40px;
  height: 30px;
  position: fixed; /* ← fixedにして常に左上固定 */
  top: 20px;
  left: 20px;
  cursor: pointer;
  z-index: 3;
  display: inline-block;
}

.menu-icon span {
  display: block;
  height: 4px;
  margin: 6px 0;
  background: #333;
  border-radius: 2px;
  transition: 0.4s;
}

/* ハンバーガーがXに変形 */
#menu-toggle:checked + .menu-icon span:nth-child(1) {
  transform: translateY(10px) rotate(45deg);
}
#menu-toggle:checked + .menu-icon span:nth-child(2) {
  opacity: 0;
}
#menu-toggle:checked + .menu-icon span:nth-child(3) {
  transform: translateY(-10px) rotate(-45deg);
}

/* オーバーレイ背景 */
.overlay {
  position: fixed;
  inset: 0;
  background: rgba(0,0,0,0.5);
  opacity: 0;
  pointer-events: none;
  transition: 0.4s;
  z-index: 1;
}

#menu-toggle:checked ~ .overlay {
  opacity: 1;
  pointer-events: auto;
}

/* メニュー本体（左から出す） */
.menu {
  position: fixed;
  top: 0;
  left: -250px; /* ← 初期位置を左へ */
  width: 250px;
  height: 100%;
  background: #fff;
  box-shadow: 2px 0 8px rgba(0,0,0,0.2);
  transition: left 0.4s ease;
  z-index: 2;
}

#menu-toggle:checked ~ .menu {
  left: 0; /* ← 開いたときは左0へ */
}

.menu ul {
  list-style: none;
  padding: 60px 20px;
}

.menu li {
  margin: 20px 0;
}

.menu a {
  text-decoration: none;
  color: #333;
  font-size: 1.2rem;
  transition: color 0.3s;
}

.menu a:hover {
  color: #007bff;
}