Concentric traingles
HTML
<div class="parent">
<div class="child1">
</div>
<div class="child2">
</div>
</div>
CSS
.parent{
position:relative;
width:400px;
height:400px;
border-radius:50%;
background-color:green;
}
.child1{
position:absolute;
width:80%;
height:80%;
border-radius:50%;
background:red;
top:10%;
left:10%;
}
.child2{
position:absolute;
width:60%;
height:60%;
border-radius:50%;
background:blue;
top:20%;
left:20%;
}