Invert Rounded Corner

HTML

<div id="outer">
    <div></div>
</div>

CSS

#outer {
 width:300px;
 height:auto; 
 overflow:hidden;
 margin:20px auto 0px;    
}

#outer div {
 width:100%;
 height:250px;
 -webkit-border-radius:50px;
 -moz-border-radius:50px;
 border-radius:50px;
 -webkit-box-shadow:0px 0px 0px 100px #39f;
 -moz-box-shadow:0px 0px 0px 100px #39f;  
 box-shadow:0px 0px 0px 100px #39f;    
}