Border radius 1

HTML

background-color: rgb(200, 0, 0);<br>opacity: 0.5;
<div class="box" id="box1"></div>
background-color: rgba(200, 0, 0, 0.5);
<div class="box" id="box2"></div>

CSS

.box {
  margin-top: 10px;
  margin-bottom: 20px;
  width: 100px;
  height: 100px;
}
#box1 {
  background-color: rgb(200, 0, 0);
  opacity: 0.5;
}
#box2 {
  background-color: rgba(200, 0, 0, 0.5);
}