CSS - HTML elements

Question

/*radio button checked*/
input[type=radio]:checked + label {
  color: $primary_color; 
} 
input[type=radio]:checked {
  &:after {
    width: 15px;
    height: 15px;
    border-radius: 15px;
    top: -2px;
    left: -1px;
    position: relative;
    background-color: $primary_color;
    content: '';
    display: inline-block;
    visibility: visible;
    border: 2px solid white;
  }
}
 
/*placeholder aka HTML label in input area <input placeholder="Email / Username" type="text" />*/
::-webkit-input-placeholder { /* Chrome/Opera/Safari */
  color: #ddd;}
::-moz-placeholder { /* Firefox 19+ */
  color: #ddd;}
:-ms-input-placeholder { /* IE 10+ */
  color: #ddd;}
:-moz-placeholder { /* Firefox 18- */
  color: #ddd;}