Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.3.0.rc.1)
// Compass (v0.13.alpha.10)
// ----

$max-mobile: 300px;

@mixin isMobile() {
    @media only screen and (max-width: $max-mobile) { @content; }
}

.reset-values {
    width: 100%;
    height: 100px;
    left: auto;
    right: auto;
    top: auto;
    bottom: auto;
}

.box {
    top: 0;
    left: 0;
    width: 25%;
    height: 50%;
    @extend .reset-values;

    @include isMobile() {
        /* mobile specific properties  */
    }
}

SassMeister-output.css

.reset-values, .box {
  width: 100%;
  height: 100px;
  left: auto;
  right: auto;
  top: auto;
  bottom: auto;
}

.box {
  top: 0;
  left: 0;
  width: 25%;
  height: 50%;
}
@media only screen and (max-width: 300px) {
  .box {
    /* mobile specific properties  */
  }
}