Selectors - CSS Reference - page 75

Name
:checked
Applies to:
Any element
Description:
Matches any user interface element that has been “toggled on,” such as a checked checkbox
or a filled radio button.
Examples:
input:checked {outline: 3px solid rgba(127,127,127,0.5);}
input[type="checkbox"]:checked {box-shadow: red 0 0 5px;}