Works with latest laravel-mix

webpack.mix.js

const mix = require('laravel-mix');
const tailwindcss = require('tailwindcss');
require('laravel-mix-purgecss');

mix.postCss('src-css/themename.css', 'css')
    .options({
        postCss: [tailwindcss('tailwind.js')],
        processCssUrls: false,
    })
    .js([
        'js/main.js',
    ], './js/themename.js');

mix.browserSync({
    proxy: 'http://example.test',
    browser: 'firefox',
    files: ['**/*.html','/css/*.css']
});

mix.disableSuccessNotifications();

if (mix.inProduction()) {
    mix.purgeCss({
        enabled: true,
        globs: [
            path.join(__dirname, 'templates/*.html'),
            path.join(__dirname, 'partials/*.html'),
            path.join(__dirname, 'partials/**/*.html'),
            path.join(__dirname, 'layouts/*.html'),
            path.join(__dirname, 'js/**.js'),
            path.join(__dirname, 'img/**.svg'),
        ],
        extensions: ['html', 'js', 'php', 'vue', 'svg'],
    })
};