less

.foo (@bg: #f5f5f5, @color: #900) {
  background: @bg;
  color: @color;
  font-size: 16px;
  font-weight: 900;
}

.unimportant {
  .foo();
}
.important {
  .foo() !important;
}

css

.unimportant {
  background: #f5f5f5;
  color: #990000;
  font-size: 16px;
  font-weight: 900;
}
.important {
  background
 
: #f5f5f5 !important;
  color: #990000 !important;
  font-size: 16px !important;
  font-weight: 900 !important;
}

less07 important