<===> input.sass
.error
  border: 1px #f00
  background-color: #fdd

.seriousError
  @extend .error
  border-width: 3px

.criticalError
  @extend .seriousError
  position: fixed
  top: 10%
  bottom: 10%
  left: 10%
  right: 10%

<===> output.css
.error, .seriousError, .criticalError {
  border: 1px #f00;
  background-color: #fdd;
}

.seriousError, .criticalError {
  border-width: 3px;
}

.criticalError {
  position: fixed;
  top: 10%;
  bottom: 10%;
  left: 10%;
  right: 10%;
}