Sass is the most mature, stable, and powerful professional grade CSS extension language in the world.
Sass is completely compatible with all versions of CSS. We take this compatibility seriously, so that you can seamlessly use any available CSS libraries.
Sass boasts more features and abilities than any other CSS extension language out there. The Sass Core Team has worked endlessly to not only keep up, but stay ahead.
Sass has been actively supported for almost 7 years by its loving Core Team.
Take a look at the Sass Guide.