You forgot to set the width of the border! Change border: red; to border:1px solid red;
Here the full code to get the circle:
.circle {
background-color:#fff;
border:1px solid red;
height:100px;
border-radius:50%;
-moz-border-radius:50%;
-webkit-border-radius:50%;
width:100px;
}<div class="circle"></div>