<div class="btn-alpha">
  <a href="#" class="btn">ボタン背景透過防止</a>
</div>
body {
   background: #FFF1D3;
}

.btn {
    min-height: 72px;
    box-sizing: border-box;
    display: grid;
    align-items: center;
    justify-content: center;
    font-size: 18px;
    font-weight: bold;
    color: #f4f4f4;
    padding: 5px;
    border-radius: 5px;
    text-decoration: none;
    background: #3b3b3b;
}

.btn:hover {
    opacity: 0.5;
}

.btn-alpha {
    border-radius: 5px;
    background: #3b3b3b;
}

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.