%animal {
    position: relative;
    background: gray;
}
.cat {
    @extend %animal;
    color: white;
}
.dog {
    @extend %animal;
    color: black;
}


/* compiled CSS */
.cat, .dog {
  position: relative;
  background: gray;
}
.cat {
  color: white;
}
.dog {
  color: black;
}