❤ An HTML, CSS and JavaScript framework for developing responsive modern web interfaces, focused on usability and minimal sizes, with all the necessary extensions.
According to the The State Of CSS 2019, Sass has the most users count and the most satisfaction percentage, so to reach a wider users base we must support Sass. On the other hand, Sass and Stylus are too much alike, it will be an easy task to migrate the code base for Stylus into Sass.
Drawbacks
Each fix/feature will be added to two different sources, doubling the maintaining efforts.
Although the two pre-processors have a very similar syntax, their compiler is completely different, this could result in a slightly different output.
"An HTML, CSS and JavaScript framework for developing responsive modern web interfaces, focused on usability and minimal sizes, with all the needed / necessary extensions."