It's totally not obvious how to make source maps work

[x-yuri]

Wrong:

new OptimizeCssAssetsPlugin({cssProcessorOptions: {map: true}})

Correct:

new OptimizeCssAssetsPlugin({cssProcessorOptions: {map: {inline: false, annotations: true}}})

Since with optimize-css-assets-webpack-plugin map is supposed to be an object. More info here.

At least this much is worth mentioning in the README,

[anlambert]

Thanks for sharing this! I was struggling on the same problem since a couple of hours.
Nevertheless, there is a typo in the annotations parameter provided to PostCSS (see https://github.com/postcss/postcss/blob/master/docs/source-maps.md)
The correct solution is:

new OptimizeCssAssetsPlugin({cssProcessorOptions: {map: {inline: false, annotation: true}}})

[iambalaam]

Everything now works correctly! Thank you