<div>
  <p>モーダル</p>
</div>
div{
  position: fixed;
  inset: 0;
  margin: auto;
  
  width:80%;
  height:80%;
  
  background-color:blue;
  
  display:flex;
  justify-content: center;
  align-items:center
}

p{
  color:#fff;
  font-size:32px;
}

External CSS

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External JavaScript

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