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CSS

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JavaScript

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HTML 

              
                <div class="header">
  <header>
    <div class="header__inner">
      <p class="header__title"><a href="http://glatchdesign.com/">Glatch</a></p>
      <p class="header__menu"><a href="#" id="js-header__menu">MENU</a></p>
    </div>
    <!--
        .header__naviにposition: fixedを指定する。
    -->
    <div class="header__navi" id="js-header__navi">
      <p class="header__navi__close"><a href="#" id="js-header__navi__close">×</a></p>
      <!-- 
          .header__navi直下でスクロールさせる為のdiv要素（.header__navi__inner）を追加。
          ここにheightとoverflowを指定する。
      -->
      <div class="header__navi__inner">
        <ul class="header__navi__list">
          <li><a href="#">MENU1</a></li>
          <li><a href="#">MENU2</a></li>
          <li><a href="#">MENU3</a></li>
          <li><a href="#">MENU4</a></li>
          <li><a href="#">MENU5</a></li>
          <li><a href="#">MENU1</a></li>
          <li><a href="#">MENU2</a></li>
          <li><a href="#">MENU3</a></li>
          <li><a href="#">MENU4</a></li>
          <li><a href="#">MENU5</a></li>
          <li><a href="#">MENU1</a></li>
          <li><a href="#">MENU2</a></li>
          <li><a href="#">MENU3</a></li>
          <li><a href="#">MENU4</a></li>
          <li><a href="#">MENU5</a></li>
          <li><a href="#">MENU1</a></li>
          <li><a href="#">MENU2</a></li>
          <li><a href="#">MENU3</a></li>
          <li><a href="#">MENU4</a></li>
          <li><a href="#">MENU5</a></li>
        </ul>
      </div>
    </div>
    </hedaer>
</div>
<!-- /.header -->
<div class="main">
  <p>Contents Area</p>
</div>
<!-- /.main -->
!

CSS 

              
                html {
  -moz-osx-font-smoothing: grayscale;
  -webkit-font-smoothing: antialiased;
  text-rendering: optimizeLegibility;
  box-sizing: border-box;
}
body {
  margin: 0;
}
body, input, button, textarea, select {
  line-height: 1.5;
  font-family: 'Open Sans', "游ゴシック体", "Yu Gothic", YuGothic, "ヒラギノ角ゴ Pro W3", "Hiragino Kaku Gothic Pro", "メイリオ", Meiryo, sans-serif;
}
*,
::before,
::after {
  box-sizing: inherit;
}
p {
  margin: 0;
}
a {
  color: #fff;
  text-decoration: none;
}
ul {
  list-style-type: none;
  margin: 0;  
  padding: 0;
}
.header__inner {
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  padding: 5px 15px;
  background-color: #26a69a;
}
.header__title > a {
  font-size: 25px;
  font-weight: bold;
}
.header__menu {
  position: absolute;
  top: 50%;
  right: 15px;
  transform: translateY(-50%);
}
.header__menu > a {
  display: block;
  padding: 8px 10px;
  font-size: 14px;
  line-height: 1;
  border: 1px solid #fff;
  border-radius: 3px;
}
.header__navi {
  position: fixed;
  top: 47px;
  left: 0;
  width: 100%;
  z-index: 100;
  text-align: center;
  
}
.header__navi__inner {
  height: 526px;
  overflow: auto;
  -webkit-overflow-scrolling: touch;
}
.header__navi__close {
  display: inline-block;
  position: fixed;
  top: 0;
  right: 0;
  margin: 0 0 0 auto;
  font-size: 30px;
  line-height: 1;
  background-color: rgba(0, 0, 0, .8);
}
.header__navi__close > a {
  display: block;
  width: 47px;
  height: 47px;
  padding: 5px 10px;
}
.header__navi__list {
  background-color: rgba(0, 0, 0, .8);
}
.header__navi__list > li:not(:first-child) {
  border-top: 1px solid gray;
}
.header__navi__list > li > a {
  display: block;
  padding: 15px 5px;
}
.main {
  display: table;
  width: 100%;
  height: 1000px;
  background-color: #f9f6e4;
}
.main p {
  display: table-cell;
  font-size: 20px;
  text-align: center;
  vertical-align: middle;
}
!

JS 

              
                var menuBtn = document.getElementById('js-header__menu');
var closeBtn = document.getElementById('js-header__navi__close');
var navi = document.getElementById('js-header__navi');

function headerMenu() {
  menuBtn.style.display = 'none';
  menuBtn.addEventListener('click', function() {
    navi.style.display = 'block';
    menuBtn.style.display = 'none';
  }, false);
  closeBtn.addEventListener('click', function() { 
    navi.style.display = 'none';
    menuBtn.style.display = 'block';
  }, false);
}

document.addEventListener('DOMContentLoaded', headerMenu, false);
!
999px

Console