Instead of using “Disabling” selectors, you should focus on writing “Enabling” selectors.

a:not(:last-child) {
  margin-bottom: 1rem;
}

or

a + a {
  margin-top: 1rem;
}

In 2012, I decided to try to learn MIT's 4-year undergraduate computer science curriculum in 12 months, without taking any classes. I was successful in passing the final exams for 33 classes and completing the required programming projects. You can view my TEDx Talk regarding the challenge, see links to all the free material I used, and see all the exams and projects I posted.