CSS

inherit css

span {
    color: blue;
}

.extra span {
    color: inherit;
}

css inherit class

.rounded_corners {
  -moz-border-radius: 8px;
  -webkit-border-radius: 8px;
  border-radius: 8px;
}

#header {
  .rounded_corners;
}

#footer {
  .rounded_corners;
}