Chris Coyi­er on PostC­SS:

We know that specs change. It hap­pens all the time. Seems weird to base a syn­tax on a non-final spec. What hap­pens when the spec changes? Do you change the lan­guage and let exist­ing code break? How is that future-proof? Or sup­port all past for­mats? Mean­ing the lan­guage isn’t real­ly based on future CSS, it’s based on any exper­i­men­tal idea that was con­sid­ered?

These have been exact­ly my thoughts since hear­ing about CSS post-proces­sors. How can peo­ple hon­est­ly believe that the code they’re writ­ing is future-proof? Sass source files cer­tain­ly are prone to “spec rot”, but the CSS they gen­er­ate isn’t (at least, inso­far as any­thing can be immune to it).

On writ­ing real CSS (again) | CSS-Tricks