Border radius 1

HTML

rgb(255,0,0)  
rgba(255,0,0,0.5)  
rgba(255,0,0,0.1)<br>
<div class="box" id="box1"></div>
<div class="box" id="box2"></div>
<div class="box" id="box3"></div>

CSS

.box {
  margin-top: 20px;
  width: 100px;
  height: 100px;
  float: left;
}
#box1 {
  background-color: rgb(255,0,0);
}
#box2 {
  background-color: rgba(255,0,0,0.5);
}
#box3 {
  background-color: rgba(255,0,0,0.1);
}