Similar to atomic CSS frameworks such as Tachyons and TailwindCSS, 2css aims to reduce every css property to just 2 letters while still reading (in your mind) as through you're writing css rules. Hopefully this reduces the memory burden when switching between frontend frameworks.
The css rules are generated by a PHP script which expects some naming conventions, then outputs a file with all the specified CSS classes.
Though the resulting file is too big to be used in production, tools such as PurgeCss are able to strip out unused rules and reduce the CSS files toa reasonable size.
For more information take a look at the 2css documentation
The benefits are faster development, less context switching and less need for cache busting, longer term the frontend styling will be more maintainable.
The downsides are additional steps required to strip out unneeded code, inflated html content and the need to strictly enfore class usage site wide.
The effects section governing animation and transitions is a bit ropey, however it's simple enough to use the excellent Animate.css library instead.