Sass Fundamentals - Extending Selectors with @extend

This topic is for the [Extending Selectors with extend][] exercise in the [Sass Fundamentals][] trail. Post any questions, corrections, or pointers you have to share with other Upcase subscribers.
[Extending Selectors with extend]: https://exercises.upcase.com/exercises/extending-selectors-with-extend
[Sass Fundamentals]: https://thoughtbot.com/upcase/sass-fundamentals