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HTML 

              
                <header>
    <img src="https://hajimete.org/wp-content/uploads/2019/06/title.png" alt="logo" />
  <span>三</span>
  </header>
  <main>
    <div id="top_contents">
      <div class="top_inner">
        <h1>我々は宇宙人だ。</h1>
        <p>
          世界征服には、どうしたってUFOは欠かせない代物である。<br />
          我々のUFOを紹介しよう！
        </p>
      </div>
    </div>
  </main>
<div id="menu" class="suru">
  <div class="menu_inner">
    <h2>MENU</h2>
    <ul>
      <li>UFOについて</li>
      <li>UFOとは？</li>
      <li>謎の飛行物体「UFO」</li>
      <li>UFOと未確認飛行物体</li>
      <li>ユーフォメーション（UFO）</li>
    </ul>
  </div>
</div>
!

CSS 

              
                body {
      margin: 0;
      padding: 0;
    }
    a {
      text-decoration: none;
      color: rgb(13, 199, 143);
      display: block;
    }
    header {
      position: fixed;
      z-index: 10;
      top:0;
      left:0;
      width: 100%;
      height: 60px;
      background: #fff;
      box-shadow: 0 2px 30px -20px rgba(0,0,0,.4);
    }
    header img {
      position: absolute;
      top: 50%;
      left: 20px;
      height: 45%;
      display:block;
      transform: translateY(-50%);
      -webkit-transform: translateY(-50%);
    }
header span {
  position: absolute;
  top: 50%;
  right: 20px;
  height: 45%;
  display:block;
  font-size: 24px;
  line-height: 1;
  font-weight: 800;
  transform: translateY(-50%);
  -webkit-transform: translateY(-50%);
}
    #top_contents {
      position: relative;
      width: 100%;
      height: 800px;
      text-align: center;
      display: block;
    }
    .top_inner {
      position: absolute;
      top: 50%;
      left: 50%;
      transform:translate(-50%, -50%);
      -webkit-transform:translate(-50%, -50%);
    }
    .top_inner a {
      display: block;
      padding: 8px 20px;
      margin-top: 30px;
      border: 1px solid rgb(13, 199, 143);
      border-radius: 12px;
      min-width: 240px;
    }
#menu {
  position: fixed;
  z-index: 9;
  top:0;
  left:0;
  width: 100%;
  height: 100%;
  background: rgba(0,0,0,.95);
  display: block;
  color: #fff;
  transition: .6s ease-in-out;
  -webkit-transition: .6s ease-in-out;
}
.menu_inner {
  position: absolute;
  top:50%;
  left: 50%;
  transform:translate(-50%, -50%);
  -webkit-transform:translate(-50%, -50%);
  display:block;
  width: 280px;
  padding-top: 60px;
  text-align: center;
}
.menu_inner h2 {
  margin-bottom: 20px;
}
.menu_inner ul {
  padding:0;
}
.menu_inner li {
  width: 100%;
  display: block;
  margin-top:20px;
  padding: 12px 0;
  border-bottom: 1px solid #fff;
}
.suru {
  transform: translateY(-100%);
  -webkit-transform: translateY(-100%);
}
!

JS 

              
                $(function() {
  $("header span").on("click", function() {
    if($(this).text() == "三") {
      $(this).text("×");
    } else {
      $(this).text("三");
    }
  });
});
!
999px

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