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CSS

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Babel includes JSX processing.

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HTML

              
                <header>
  <div class="div1">
    div1
  </div>
  <div class="div2">
    div2
  </div>
  <div class="div3">
    div3
  </div>
  <div class="div4">
    div4
  </div>
  <div class="div5">
    div5
  </div>
</header>
              
            
!

CSS

              
                /* 使用している画像のサイズは1200x600 */
header {
  width: 1200px;
}

div {
  border: solid;
  margin: 10px;
}

/* OKパターン */
.div1 {
  widht: 100%;
  background-image: url(https://source.unsplash.com/random/1200x600);
  background-size: cover;
  /* (画像の横幅 / 画像の縦幅) * 100 */
  padding-top: 50%;  
}

/* background-imageのみ指定 */
/*  →要素の高さが0なので背景画像は表示されない */
.div2 {   
  widht: 100%;
  background-image: url(https://source.unsplash.com/random/1200x600);  
}

/* 要素に「div3」というテキストが入っている */
/*  →テキスト高さ分だけ背景画像が表示される */
.div3 {
  widht: 100%;
  background-image: url(https://source.unsplash.com/random/1200x600);    
}

/* 要素の高さを指定 */
/*  →要素の高さ分だけ背景画像が表示される */
.div4 {
  widht: 100%;
  height: 200px;
  background-image: url(https://source.unsplash.com/random/1200x600);    
}

/* padding-topに「(画像の横幅 / 画像の縦幅) * 100」の値を%で指定 */
/*  →比率を保ったままブラウザの大きさに合わせて背景画像が表示される */
/*   しかし、background-sizeにcoverを指定していないため、back5というテキストの高さ分、背景画像が繰り返し表示される */
.div5 {
  widht: 100%;
  background-image: url(https://source.unsplash.com/random/1200x600);
  /* (画像の横幅 / 画像の縦幅) * 100 */
  padding-top: 50%;
}


              
            
!

JS

              
                
              
            
!
999px

Console