<div class="box1">divで一つ目の要素を作ったよ!</div>
  <div class="box2">これが2つ目で「box2」という識別子!</div>
  <div class="box3">「box3」のクラスで作成したよ!</div>
.box1 {
  background: skyblue;
  width: 100px;
  height: 100px;
}

.box2 {
  background: yellow;
  width: 100px;
  height: 100px;
}
.box3 {
  background: red;
  width: 100px;
  height: 100px;
}

External CSS

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External JavaScript

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