<div id="box" class="okragle-wszystkie-rogi"> </div>

#box {
	width: 100px;
	height: 100px;
	background: red;
}

.okragle-wszystkie-rogi {
	-webkit-border-radius: 20px; /* Safari, Chrome */
	-khtml-border-radius: 20px;	/* Konqueror */
	-moz-border-radius: 20px; /* Firefox */
	border-radius: 20px;
}