Code examples made for https://cnpagency.com/blog/trim-hidden-bulk-in-your-sass-extend-directives/. Don't delete this if that's still up (and it is as of 2022 Jul 2).

extend-example-1.css

.button, .big-button {
   display: inline-block;
   padding: 1em;
}
.big-button {
   font-size: 1.4em;
}

extend-example-1.scss

.button {
   display: inline-block;
   padding: 1em;
}
.big-button {
   @extend .button;
   font-size: 1.4em;
}

extend-example-2.css

.button, .big-button, .slideshow .js-plugin-link {
  display: inline-block;
  padding: 1em;
}

.big-button {
  font-size: 1.4em;
  background-color: red;
}

form .button, form .big-button, form .slideshow .js-plugin-link, .slideshow form .js-plugin-link {
  background-color: blue;
}

.slideshow .js-plugin-link {
  position: absolute;
  bottom: 5%;
}

.sidebar .button, .sidebar .big-button, .sidebar .slideshow .js-plugin-link, .slideshow .sidebar .js-plugin-link {
  background-color: blue;
}
.sidebar .button:after, .sidebar .big-button:after, .sidebar .slideshow .js-plugin-link:after, .slideshow .sidebar .js-plugin-link:after {
  content: "\2192";
}

extend-example-2.scss

.button {
  display: inline-block;
	padding: 1em;
}
.big-button {
	@extend .button;
	font-size: 1.4em;
	background-color: red;
}
form .button {
	background-color: blue;
}
.slideshow .js-plugin-link {
	@extend .button;
	position: absolute;
	bottom: 5%;
}
.sidebar .button {
	background-color: blue;

	&:after {content: "\2192";}
}

extend-example-3.css

.button, .big-button, .slideshow .js-plugin-link {
  display: inline-block;
  padding: 1em;
}

.big-button {
  font-size: 1.4em;
  background-color: red;
}

form .button {
  background-color: blue;
}

.slideshow .js-plugin-link {
  position: absolute;
  bottom: 5%;
}

.sidebar .button {
  background-color: blue;
}
.sidebar .button:after {
  content: "\2192";
}

extend-example-3.scss

%button {
  display: inline-block;
	padding: 1em;
}
.button {
	@extend %button;
}
.big-button {
	@extend %button;
	font-size: 1.4em;
	background-color: red;
}
form .button {
	background-color: blue;
}
.slideshow .js-plugin-link {
	@extend %button;
	position: absolute;
	bottom: 5%;
}
.sidebar .button {
	background-color: blue;

	&:after {content: "\2192";}
}