<header>
  <div>Title</div>
  <nav class="nav">
    <ul>
      <li><a href="#">Home</a></li>
      <li><a href="#">Menu</a></li>
      <li><a href="#">News</a></li>
      <li><a href="#">Contact</a></li>
    </ul>
  </nav>
  <button type="button" id="navbtn"></button>
</header>
@media (max-width:767px){
  html,body{
    overflow-x: hidden;
  }
  header div{
    display: none;
  }
  .nav{
    position: fixed;
    top: 0;
    left: -100%;
    width: 100%;
    height: 100vh;
    background: rgba(0,0,0,0.9);
    color: #ffffff;
    display: flex;
    justify-content: center;
    align-items: center;
    /* 左からスライド */
    transition: left 0.5s;
  }
  .nav ul{
    list-style:none;
  }
  .nav li:not(:last-child){
    margin-bottom: 30px;
  }
  .nav a{
    color: #ffffff;
  }
}
/* ナビゲーションボタン(開く) */
#navbtn{
  position: fixed;
  top: 15px;
  left: 15px;
  padding: 0;
  outline: none;
  border: none;
  background: none;
  width: 30px;
  height: 20px;
  cursor: pointer;
}

#navbtn::before,
#navbtn::after{
  content : '';
  display: block;
  height: 1px;
  background-color: #333333;
  transform: translateY(10px);
  transition: 0.3s ease-in-out;
}

#navbtn::before{
  transform: translateY(-10px);
  box-shadow: 0 10px #333333;
}

/* ナビゲーションボタン(閉じる) */
.open #navbtn{
  z-index: 100;
}
.open #navbtn::before{
  transform: rotate(-45deg);
  box-shadow: none;
}
.open #navbtn::after{
  transform: rotate(45deg);
  box-shadow: none;
}
/* ナビゲーションメニュー 開いた時*/
.open .nav{
  left: 0;
}

@media(min-width: 768px){
  /* PCではボタン非表示 */
  #navbtn{
    display:none;
  }
  /* ヘッダーは横並び */
  header{
    display: flex;
    align-items: center;
  }
  /* メニューを右に寄せる */
  .nav{
    /* 右寄せ */
    margin-left: auto;
  }
  .nav ul{
    display: flex;
    list-style:none
  }
  .nav ul li:not(:first-child){
    margin-left:20px;
  }
  .nav ul li a { 
    color: #000000;
  }
}
document.getElementById("navbtn").onclick = function(){
  document.querySelector('html').classList.toggle('open');
}

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