matrix-spec/postcss.config.js

// Remove unused CSS selectors.
const purgecss = require('@fullhuman/postcss-purgecss')({
    // Use stats generated by Hugo.
    content: [ './hugo_stats.json' ],
    defaultExtractor: (content) => {
        let els = JSON.parse(content).htmlElements;
        return els.tags.concat(els.classes, els.ids);
    }
});

module.exports = {
     plugins: [
         ...(process.env.HUGO_ENVIRONMENT === 'production' ? [ purgecss ] : [])
     ]
 };