example 9a.scss

%post-skeletor{ 
    font:{ size: 12; weight: 700;}
    box-sizing: border-box;
    padding: 5px;
    margin: 10px;
    border: 1px solid whitesmoke;
}

.post-class {
    font:{ size: 12; weight: 700;}
    box-sizing: border-box;
    padding: 5px;
    margin: 10px;
    border: 1px solid whitesmoke;
}

.post1--extends__placeholder {
    @extend %post-skeletor;
    background: yellow;
}

.post1--extends_class{
    @extend .post-class;
    background: yellow;
}

example 9b.css

.post1--extends__placeholder {
  font-size: 12;
  font-weight: 700;
  box-sizing: border-box;
  padding: 5px;
  margin: 10px;
  border: 1px solid whitesmoke;
}

.post-class, .post1--extends_class {
  font-size: 12;
  font-weight: 700;
  box-sizing: border-box;
  padding: 5px;
  margin: 10px;
  border: 1px solid whitesmoke;
}

.post1--extends__placeholder {
  background: yellow;
}

.post1--extends_class {
  background: yellow;
}