Svelte style preprocessor using PostCSS

rollup.config.js

import svelte from 'rollup-plugin-svelte';
import resolve from 'rollup-plugin-node-resolve';
import commonjs from 'rollup-plugin-commonjs';
import buble from 'rollup-plugin-buble';
import uglify from 'rollup-plugin-uglify';
import postcss from 'postcss';
import postcssImport from 'postcss-import';
import postcssCssnext from 'postcss-cssnext';

const production = !process.env.ROLLUP_WATCH;

export default {
	input: 'src/main.js',
	output: {
		sourcemap: true,
		format: 'iife',
		file: 'public/bundle.js'
	},
	name: 'app',
	plugins: [
		svelte({
			// enable run-time checks when not in production
			dev: !production,
			// we'll extract any component CSS out into
			// a separate file — better for performance
			css: css => {
				css.write('public/bundle.css');
			},

			// this results in smaller CSS files
			cascade: false,

			preprocess: {
				style: ({ content, attributes }) => {
					if (attributes.type !== 'text/postcss') return;

					return new Promise((fulfil, reject) => {
						const plugins = [postcssImport, postcssCssnext];
						const from = 'src';

						postcss(plugins)
							.process(content, {
								from,
								map: {
									inline: false
								}
						})
						.then(result => {
							fulfil({
								code: result.css.toString(),
								map: result.map.toString()
							});
						})
						.catch(err => reject(err));
					});
				}
			}
		}),

		// If you have external dependencies installed from
		// npm, you'll most likely need these plugins. In
		// some cases you'll need additional configuration —
		// consult the documentation for details:
		// https://github.com/rollup/rollup-plugin-commonjs
		resolve(),
		commonjs(),

		// If we're building for production (npm run build
		// instead of npm run dev), transpile and minify
		production && buble({ exclude: 'node_modules/**' }),
		production && uglify()
	]
};

style: ({ content, attributes }) => {
    if (attributes.type !== 'text/postcss') {
        return;
    };

    // Plugins List: 
    // https://github.com/postcss/postcss/blob/master/docs/plugins.md
    const plugins = [
        require('postcss-import'),
        require('postcss-cssnext')
    ];
    return postcss(plugins)
        .process(content, { 
            from: 'src', 
            map: { inline: false } 
        })
        .then((result) => ({
            code: result.css.toString(),
            map: result.map.toString()
        }));
}