Invert rounded corner in CSS?

In modern browsers, you can use mask-image:

#aux-container {
  width: 100px;
  height: 100px;
  background: #f00;
  -webkit-mask-image: radial-gradient(circle 10px at 0 0, transparent 0, transparent 20px, black 21px);
}

<div id="aux-container"></div>

http://jsbin.com/eViJexO/1/

Additionally, take a look at http://www.html5rocks.com/en/tutorials/masking/adobe/, which describes how to achieve similar result using mask-box-image.