Sass is handy, but can't do everything. Learn how to use PostCSS to create a custom CSS preprocessor that compiles Sass syntax but also adds extra features.
Thomas Greco introduces you to postcss-cssnext, the plugin pack that lets you write the CSS of the future and makes it work in the browsers of today
Pavels Jelisejevs looks at using PostCSS plugins to achieve some of the new upcoming CSS selector and media query features.
Craig describes how to implement the preprocessing functionality you require using significantly faster, modular code.