We want to make it easier to write awesome CSS that works great in as many browsers as possible. Some of these browsers are a bit older, others don't yet exist.
We will not place undue burden on anyone writing or maintaining CSS.
We will only add a plugin to postcss-preset-env when the CSS WG invites implementations for the relevant features.
We will not create plugins with features (or add features to plugins) that look and feel native but don't have a specification.
We will make it clear that non-standard, pre-processor features are just that.
We will only implement that which we know is correct and avoid side effects.