An example of bad coding practice - deeply nested Sass leading to over-qualified CSS.

SassMeister-input.scss

// ----
// Sass (v3.3.4)
// Compass (v1.0.0.alpha.18)
// ----

// This code sucks.

.module-container {
  background-color: red;

  .module-sub-container {
    background-color: yellow;

    .feature1 {
      background-color: orange;

      .feature2-container {
        background-color: white;

        span {
          background-color: green;
        }
      }
      .feature3 {
        background-color: transparent;

        &.option1 {
          background-color: blue;

          .option2 {
            background-color: purple;
          }
        }
        .option3 {
          background-color: black;
        }
      }
    }
  }
}

// This is how you might re-write it in a more efficient, reusable way

/* --------------------------------------------- */

.module_container {
  background-color: red;
}

.module_sub-container {
  background-color: yellow;
}
  
.module_feature1 {
  background-color: orange;
}

.module_feature2-container {
  background-color: white;
}

.module_feature2-element { // used to be `span`
  background-color: green;
}
 
.module_feature3 {
  background-color: transparent;
  
  &--option1 {
    background-color: blue;
  }
  
  &--option2 {
    background-color: purple;
  }
  
  &--option3 {
    background-color: black;
  }
}

SassMeister-output.css

.module-container {
  background-color: red;
}
.module-container .module-sub-container {
  background-color: yellow;
}
.module-container .module-sub-container .feature1 {
  background-color: orange;
}
.module-container .module-sub-container .feature1 .feature2-container {
  background-color: white;
}
.module-container .module-sub-container .feature1 .feature2-container span {
  background-color: green;
}
.module-container .module-sub-container .feature1 .feature3 {
  background-color: transparent;
}
.module-container .module-sub-container .feature1 .feature3.option1 {
  background-color: blue;
}
.module-container .module-sub-container .feature1 .feature3.option1 .option2 {
  background-color: purple;
}
.module-container .module-sub-container .feature1 .feature3 .option3 {
  background-color: black;
}

/* --------------------------------------------- */
.module_container {
  background-color: red;
}

.module_sub-container {
  background-color: yellow;
}

.module_feature1 {
  background-color: orange;
}

.module_feature2-container {
  background-color: white;
}

.module_feature2-element {
  background-color: green;
}

.module_feature3 {
  background-color: transparent;
}
.module_feature3--option1 {
  background-color: blue;
}
.module_feature3--option2 {
  background-color: purple;
}
.module_feature3--option3 {
  background-color: black;
}