<div class="btn">
  <a href="">
    背景色が薄くなります
  </a>
</div>
body {
    text-align: center;
    margin-top: 100px;
}

a {
   text-decoration: none;
}

.btn a {
    display: inline-block;
    padding: 30px 60px;
    color: #fff;
    background-color: rgba(51, 51, 51, 0.7);
}

External CSS

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External JavaScript

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