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<div class="rect">&nbsp;<span class="circle"></span></div>
body{
    background:green;
}

.rect
{
    height: 100px;
    width: 100px;
    background:rgba(0,0,0,0.5);
    position:relative;
    margin-top:100px;
    margin-left:100px;
}
.circle{
    display:block;
    width: 100px;
    height: 50px;
    top:-50px;
    left:0;
    overflow:hidden;
    position:absolute;
}
.circle:after{
    content:'';
    width: 100px;
    height: 100px;
    -moz-border-radius: 100px;
    -webkit-border-radius: 100px;
    border-radius: 100px;
    background:rgba(0,0,0,0);
    position:absolute;
    top:-100px;
    left:-40px;
    border:40px solid rgba(0,0,0,0.5);
}