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HTML

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HTML preprocessors can make writing HTML more powerful or convenient. For instance, Markdown is designed to be easier to write and read for text documents and you could write a loop in Pug.

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Adding Classes

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About the <head>

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Insecure Resource

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CSS

About CSS Preprocessors

CSS preprocessors help make authoring CSS easier. All of them offer things like variables and mixins to provide convenient abstractions.

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CSS Base

About CSS Base

It's a common practice to apply CSS to a page that styles elements such that they are consistent across all browsers. We offer two of the most popular choices: normalize.css and a reset. Or, choose Neither and nothing will be applied.

Vendor Prefixing

About Vendor Prefixing

To get the best cross-browser support, it is a common practice to apply vendor prefixes to CSS properties and values that require them to work. For instance -webkit- or -moz-.

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JavaScript

About JavaScript Preprocessors

JavaScript preprocessors can help make authoring JavaScript easier and more convenient.

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Babel includes JSX processing.

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Packages

Add Packages

Search for and use JavaScript packages from npm here. By selecting a package, an import statement will be added to the top of the JavaScript editor for this package.

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Using packages here is powered by esm.sh, which makes packages from npm not only available on a CDN, but prepares them for native JavaScript ESM usage.

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HTML 

              
                <body>
  <header id="top">
    <p>header</p>
  </header>
  <main>
    <div class="top-visual">
      <h1>top-visual</h1>
    </div>
    <div class="sec"></div>
  </main>
  <a class="btn" href="#top">to top</a>
  <footer>
    <p>footer</p>
  </footer>
</body>
!

CSS 

              
                *{
  margin: 0;
}
body{
  position: relative;
}
header{
  width: 100%;
  height: 80px;
  background-color: #333;
}
header p{
  line-height: 80px;
  text-align: center;
  color: #fff;
}
.top-visual{
  width: 100%;
  height: 400px;
  background-color: cadetblue;
  position: relative;
}
.top-visual h1{
  font-size: 30px;
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}
.sec{
  background-color: aquamarine;
  height: 2000px;
}
.btn{
  position: fixed;
  bottom: 0;
  right: 10%;
  text-decoration: none;
  background-color: red;
  color: #fff;
  padding: 20px 10px;
  border-radius: 50%;
  opacity: 0;
  transition: opacity .5s;
}
.btn.active{
  opacity: 1;
}
/* 追記 */
.btn.stop{
  position: absolute;
  bottom: 200px;
}
footer{
  height: 200px;
  background-color: blue;
}
footer p{
  line-height: 200px;
  text-align: center;
  color: #fff;
}
!

JS 

              
                //スクロールした時に処理を実行
window.addEventListener('scroll', function(){
  //トップへ戻るボタンを取得
  let topBtn = document.querySelector('.btn');
  
  //画面上部からトップビジュアル下の位置取得
  const topVisual = document.querySelector('.top-visual').getBoundingClientRect().bottom;

  //トップビジュアル下の位置より下にスクロールされたらactiveクラウを付与
  if(topVisual <= 0){
    topBtn.classList.add('active');
  } else {
    //スクロールが200px未満のときactiveクラスを外す
    topBtn.classList.remove('active');
  }
  //ドキュメントの高さ
  const scrollHeight = document.body.clientHeight;

  //スクロール位置
  const scrollPosition = window.pageYOffset;

  //windowの高さ
  const windowHeignt = window.innerHeight;

  //footer取得
  const footer = document.querySelector('footer');
  console.log(footer.getBoundingClientRect().top)
  //footerの高さ
  const footerHeight = footer.offsetHeight;
  if(scrollHeight - scrollPosition - windowHeignt <= footerHeight){
    topBtn.classList.add('stop');
  } else {
    topBtn.classList.remove('stop');
  }
});
!
999px

Console