i-rwd-css4.scss

// variables.scss
$screen-phone: 767px;
$screen-rest: $screen-phone + 1px;
 
// mixins.scss
$is_phone: 0;
@mixin responsive {
  $is_phone: 0;
  @media screen and (min-width: $screen-rest) {
    @content;
  }
  $is_phone: 1;
  @content;
}
 
@mixin r($rule, $phone, $rest) {
  @if $is_phone == 1 {
    #{$rule}: $phone;
  }
  @else {
    #{$rule}: $rest;
  }
}
 
// view or component
.foo {
  margin: 10px;
  @include responsive {
    @include r(padding, 10px, 20px);
    @include r(content, "phone", "the rest");
  }
}

o-rwd-css4.css

.foo {
  margin: 10px;
  padding: 10px;
  content: "phone";
}
@media screen and (min-width: 768px) {
  .foo {
    padding: 20px;
    content: "the rest";
  }
}