At this point there are multiple "classless" CSS "frameworks" posted on HN daily and subsequently hitting frontpage. This fad has become quite boring, no offense to the author of this particular framework.
What's so hard about just throwing a few lines of CSS in a file and adding a link element to the <head>? You can get the same amount of mileage by spending 10 minutes writing generic rules with the benefit of knowing exactly what part of the page is affected by which rule (more valuable to me than the instant gratification of classless frameworks).
This particular framework is kind of a shortcut if you don't want to start from a blank css file and want to throw a web app together quickly.
Also, there are differences here from most other classless css frameworks because this mixes classless, atomic css and some swiftui like helpers for maximum html impact
Classless css helps you get going quickly with just html, think a <form> with <input> and <label> and everything looks decent. You haven't written any custom css.
Atomic css helps you add some customization, things like padding, margin, colors, borders as classes, again without writing any custom css, you are still in the same html file.
Swiftui helpers are <vstack>, <hstack> and <spacer />. you don't have to mess with bootstrap grids or custom css grids, just slap your elements in an hstack and they are now laid out horizontally as one example.
To sum it up, I would say the mix of these different approaches to css help you ultimately write no custom css yourself, you can focus on writing html, a few classes and that's it.
That's why I'm leaning toward TailwindCSS in almost every project. It's just a collection of classes. No prebuilt components, no big decisions being made for you, etc. It feels a bit freeing after 15+ years of CSS.
What's so hard about just throwing a few lines of CSS in a file and adding a link element to the <head>? You can get the same amount of mileage by spending 10 minutes writing generic rules with the benefit of knowing exactly what part of the page is affected by which rule (more valuable to me than the instant gratification of classless frameworks).