RGBA Illustration

<style>
.red { background-color: red; z-Index: 3;}
.blue { background-color: blue; z-Index: 1;}
.green { background-color: green; z-Index: 2;}
.yellow { background-color: white; }
div {
    width: 120px; height: 120px;
    position: absolute;
    top: 16px; left: 16px;
    border: 1px solid black;
    opacity: 80%;
}
div:nth-of-type(2) { top: 32px; left: 32px;}
div:nth-of-type(3) { top: 48px; left: 48px;}
div:nth-of-type(4) { top: 64px; left: 64px;}
</style>
<div class="red"></div>
<div class="green"></div>
<div class="blue"></div>
<div class="yellow"></div>