fieldset {
  border: 1px solid #000;

  &:disabled {
    border-color: red;
  }

  :disabled {
    opacity: 0.5;
  }
}

/* *** */

fieldset {
  border: 1px solid #000;
}

fieldset:disabled {
  border-color: red;
}

fieldset :disabled {
  opacity: 0.5;
}

.foo {
  color: red;
  
  & & & {
    color: blue;
  }
}

/* *** */

.foo {
  color: red;
}

.foo .foo .foo {
  color: blue;
}