Frontend developers have been using PostCSS for the transition period until this is fully adopted (which is a library set similar to Babel for adopting future JS, as well as experimenting with new CSS language level ideas):

https://github.com/MadLittleMods/postcss-css-variables

There’s also plugins for nesting and mixins if you want a more complete SASS replacement which seems to be the toolset the industry is moving towards.

The most popular PostCSS library is autoprefixer which is also helping with the transition to future CSS without all of the browser baggage. https://github.com/postcss/autoprefixer