https://astexplorer.net/#/gist/07b46ae22752d58d0beff81681e46033/d808030b73bec37edabc22f5b1968d573645482a uses csstree-2.0.1. The latest version on the 2.x branch of csstree is version 2.3.1. Maybe even better would be to go for 3.0.0 instead, which was released just a month ago.

(For clarity: this issue is not about allowing users to switch versions, which is tracked in #668. This issue is about updating to the latest version).