Result

よくあるオフキャンバスナビゲーションの類ではなく、その場で開閉するパターンです。

css

.nav {/*ナビ全体を包括するコンテナの装飾*/
  overflow: hidden;
  position: absolute;
  left: 50%;
  top: 50%;
  width: auto;
  height: 90px;
  margin-top: -45px;
  background: #fff;
  border-radius: 5px;
  transform: translate3d(-50%, 0, 0);
  box-shadow: 0 10px 35px rgba(0, 0, 0, 0.2);
}
.nav__cb {/*開閉を担うcheckbox部分。pointer-events: none;がキモ*/
  z-index: -1000;
  position: absolute;
  left: 0;
  top: 0;
  opacity: 0;
  pointer-events: none;
}
.nav__content {/*イージングアニメーション設定*/
  position: relative;
  width: 90px;
  height: 100%;
  transition: width 1s cubic-bezier(0.49, -0.3, 0.68, 1.23);
}
.nav__cb:checked ~ .nav__content {/*checkboxにチェックが入ったら実行*/
  transition: width 1s cubic-bezier(0.48, 0.43, 0.29, 1.3);
  width: 410px;
}
.nav__items {/*コンテナ内部のナビゲーション*/
  position: relative;
  width: 410px;
  height: 100%;
  padding-left: 20px;
  padding-right: 110px;
  list-style-type: none;
  font-size: 0;
}
.nav__item {/*ナビゲーション各項目*/
  display: inline-block;
  vertical-align: top;
  width: 70px;
  text-align: center;
  color: #6C7784;
  font-size: 14px;
  line-height: 90px;
  font-family: Helvetica, Arial, sans-serif;
  font-weight: bold;
  perspective: 1000px;
  transition: color 0.3s;
  cursor: pointer;
}
.nav__item:hover {
  color: #00bdea;
}
.nav__item-text {/*開閉時のアニメーション設定*/
  display: block;
  height: 100%;
  transform: rotateY(-70deg);
  opacity: 0;
  transition: transform 0.7s cubic-bezier(0.48, 0.43, 0.7, 2.5), opacity 0.7s;
}
.nav__cb:checked ~ .nav__content .nav__item-text {
  transform: rotateY(0);
  opacity: 1;
  transition: transform 0.7s cubic-bezier(0.48, 0.43, 0.7, 2.5), opacity 0.2s;
}
.nav__item:nth-child(1) .nav__item-text {
  transition-delay: 0.3s;
}
.nav__cb:checked ~ .nav__content .nav__item:nth-child(1) .nav__item-text {
  transition-delay: 0s;
}
.nav__item:nth-child(2) .nav__item-text {
  transition-delay: 0.2s;
}
.nav__cb:checked ~ .nav__content .nav__item:nth-child(2) .nav__item-text {
  transition-delay: 0.1s;
}
.nav__item:nth-child(3) .nav__item-text {
  transition-delay: 0.1s;
}
.nav__cb:checked ~ .nav__content .nav__item:nth-child(3) .nav__item-text {
  transition-delay: 0.2s;
}
.nav__item:nth-child(4) .nav__item-text {
  transition-delay: 0s;
}
.nav__cb:checked ~ .nav__content .nav__item:nth-child(4) .nav__item-text {
  transition-delay: 0.3s;
}
.nav__btn {/*開閉ボタン設定*/
  position: absolute;
  right: 0;
  top: 0;
  width: 90px;
  height: 90px;
  padding: 36px 31px;
  cursor: pointer;
}
.nav__btn:before, .nav__btn:after {/*開閉ボタン設定*/
  content: "";
  display: block;
  width: 28px;
  height: 4px;
  border-radius: 2px;
  background: #096DD3;
  transform-origin: 50% 50%;
  transition: transform 1s cubic-bezier(0.48, 0.43, 0.29, 1.3), background-color 0.3s;
}
.nav__btn:before {
  margin-bottom: 10px;
}
.nav__btn:hover:before, .nav__btn:hover:after {
  background: #00bdea;
}
.nav__cb:checked ~ .nav__btn:before {/*開閉ボタンのエフェクト。checkboxにチェックが入ったら実行される*/
  transform: translateY(7px) rotate(-225deg);
}
.nav__cb:checked ~ .nav__btn:after {
  transform: translateY(-7px) rotate(225deg);
}

html

<nav class="nav">
  <input type="checkbox" class="nav__cb" id="menu-cb"/>
  <div class="nav__content">
    <ul class="nav__items">
      <li class="nav__item">
        <span class="nav__item-text">
          Home
        </span>
      </li>
      <li class="nav__item">
        <span class="nav__item-text">
          Works
        </span>
      </li>
      <li class="nav__item">
        <span class="nav__item-text">
          About
        </span>
      </li>
      <li class="nav__item">
        <span class="nav__item-text">
          Contact
        </span>
      </li>
    </ul>
  </div>
  <label class="nav__btn" for="menu-cb"></label>
</nav>

via

Pure CSS Header Menu