#wrapper {
    width: 300px; height: 300px;
    border-radius: 100px;
    overflow: hidden;
    position: absolute; /* this breaks the overflow:hidden in Chrome/Opera */
    -webkit-mask-image: url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAIAAACQd1PeAAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAAA5JREFUeNpiYGBgAAgwAAAEAAGbA+oJAAAAAElFTkSuQmCC); /* this fixes the overflow:hidden in Chrome/Opera */
}

#box {
    width: 300px; height: 300px;
    background-color: #cde;
}