Which property makes round corner to an element?

The Answers  is : CSS3 border-radius property.

For example on border-radius property. i.e.

.ui-widget-header 

{

    font-family: "TradeGothicLTPro-BdCn20" !important;

    background: #178093 !important;

    border-width: 0 0 1px 0;

    border-radius: 0 !important;

}