I'm thinking about a different approach: The user configures this plugin like this:

{
  source: "...",
  target: "...",
  cssnext: true,
  ignore: ["rem"],
  polyfill: ["custom-properties"]
}

With this configuration, we will throw an error whenever we encounter a feature that is not supported by one of the selected browsers (by your browserslist) unless it is ignored or polyfilled:

• ignored features will just be ignored
• polyfill features will be polyfilled with a prefix or a polyfill (one of this list of postcss plugins)

This can be accomplished by:

• adding all polyfill CSS plugins chosen by the user as plugins to the pipeline
• and as a last plugin add doiuse with the provided ignore list.

So this is pretty straightforward and will probably result in a pretty nice workflow: You just start with no configuration except cssnext: true. Whenever we add a feature that is not supported by one of our chosen browsers, our pipeline will throw an error. We have three choices (that should probably be presented to us on the CLI – powered by doiuse's onFeatureUsage):

• remove the usage of the feature
• ignore that feature
• polyfill that feature

What do you think, @FND ?

I have a working prototype:

let postcss = require('postcss')
let doiuse = require('doiuse')
let fs = require('fs')
let postcssCustomProperties = require('postcss-custom-properties')

let plugins = [
        // first, do the import stuff (skipped in this prototype)
        // then, add all transforms chosen by the user
        postcssCustomProperties({
                preserve: false
        }),
        // finally, run doiuse with the provided browsers and ignored
        // features
        doiuse({
                browsers: ['ie >= 8'],
                ignore: ['rem'],
                onFeatureUsage: function(usageInfo) {
                        console.error(usageInfo.message)
                }
        })
]

fs.readFile('src/app.css', (err, css) => {
        postcss(plugins)
                .process(css, { from: 'src/app.css', to: 'dest/app.css' })
                .then(result => {
                        fs.writeFile('dest/app.css', result.css, () => true)
                })
})

Tested with this CSS:

:root {
        --color: red;
}
  
.a {
        font-size: 1rem;
        color: var(--color);
}

it runs without any warnings and produces:

.a {
        font-size: 1rem;
        color: red;
}

If you remove either the postcss-custom-properties transform or the ignore entry, it will log an error.