The :default pseudo selector will match the default in a group of associated elements, such as the radio button in a group of buttons that is selected by default, even if the user has selected a different value.
Examples include the default submit button in a group of buttons, the default option from a popup menu, the radio button that has the checked attribute set in the HTML (as seen in supporting browsers), and the checkboxes that are by default checked.
Different browsers have different levels of support. Firefox provides full support. Webkit browsers support default on buttons but not on radio or checkboxes, and Opera is the inverse, supporting it on radio buttons and checkboxes, but not on the submit.