Say what you want about Tailwind's usefulness, but the author's claim in the section titled "Rule sets" is objectively false. In the Tailwind documentation they mention the @apply directive which allows you to use Tailwind's styles in your primary stylesheet.

Here is the portion of the documentation which talks about this: https://tailwindcss.com/docs/functions-and-directives#apply-...

In my own project, I moved to Tailwind recently and found @apply to be very useful. I experiment with the inline styles and move them to a CSS class when (and if) needed. Additionally, the Tailwind CLI translates the states like hover: and active: as well. I personally have found it super convenient.