<div class="focus">
    <a tabindex="0">ボタン</a>
</div>
.focus {
  background: white; 
  height: 100vh;
  width: 100%;
  text-align:center;
}
a {
  margin:80px auto;
  display: inline-block;
  padding: 20px 40px 20px 40px;
  color: aliceblue;
  font-weight: bold;
  background: #333;
}
.focus:has(a:focus){
  background: #a2e322; //キーボードでフォーカスすれば緑に変わる
}
.focus:has(a:hover){
  background: #a2e322; //キーボードでフォーカスすれば緑に変わる
}

External CSS

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External JavaScript

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