CodePen Embed - GRbMvLy

<div class="container">
  <button>ボタン</button>  
</div>

.container {
  display: flex;
  align-items: center;
  justify-content: center;
  height: 100vh;
}

button:focus {
  outline: 0;
  box-shadow: 0 0 3px 1px red;   
}

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