Details

Always use placeholder selectors in @extend.

Bad: extending a class

.fatal {
  @extend .error;
}

Good: extending a placeholder

.fatal {
  @extend %error;
}

Using a class selector with the @extend directive usually results in more generated CSS than when using a placeholder selector. Furthermore, Sass specifically introduced placeholder selectors in order to be used with @extend.

See Mastering Sass extends and placeholders.

If you want to prevent the use of the @extend directive entirely, see the ExtendDirective linter.