<div class="tr-1">
  <p>透過なしの背景色</p>
</div>
<div class="tr-07">
  <p>第4引数に0.7指定</p>
</div>
<div class="tr-04">
  <p>第4引数に0.4指定</p>
</div>
<div class="tr-01">
  <p>第4引数に0.1指定</p>
</div>
div{
  text-align:center;
}

.tr-1{
  background-color:rgba(255,0,0,1);
}

.tr-07{
  background-color:rgba(255,0,0,0.7);
}

.tr-04{
  background-color:rgba(255,0,0,0.4);
}

.tr-01{
  background-color:rgba(255,0,0,0.1);
}

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