From CSS Wizardry:

> The problems with `@extend` are manifold, but to summarise:
> - It’s actually worse for performance than mixins are. Gzip favours repetition, so CSS files with greater repetition (i.e. mixins) achieve a greater compression delta.
> - It’s greedy. Sass’ `@extend` will extend every instance of a class that it finds, giving us crazy-long selector chains that look like this.
> - It moves things around your codebase. Source order is vital in CSS, so moving selectors around your project should always be avoided.
> - It obscures the paper-trail.`@extend` hides a lot of complexity in your Sass that you need to unpick more gradually, whereas the multiple class approach puts all of the information front-and-center in your markup.

## See

- Mixins Better for Performance
- When to use @extend; when to use a mixin
- Extending silent classes in Sass