官方例子
通过style标签

// webpack.config.js
const path = require('path');
const HTMLWebpackPlugin = require('html-webpack-plugin');
const webpack = require('webpack');

module.exports = {
    entry: {
        index: './src/index.js',
    },
    output: {
        path: path.resolve(__dirname, 'dist'),
        filename: '[name].bundle.js',
    },
    module: {
        rules: [
            {
                test: /\.css$/,
                use: [{loader:'style-loader'},{loader:'css-loader'}]
            }
        ]
    },
    plugins: [
        new HTMLWebpackPlugin({
          title: 'css'
        })
    ]
};
// index.js
import './css/base.css';
import './css/common.css';

// base.css

通过link引入css,不能指定路径

const path = require('path');
const HTMLWebpackPlugin = require('html-webpack-plugin');
const webpack = require('webpack');

module.exports = {
    entry: {
        index: './src/index.js',
    },
    output: {
        path: path.resolve(__dirname, 'dist'),
        filename: '[name].bundle.js',
    },
    module: {
        rules: [
            {
                test: /\.css$/,
                use: [{loader:'style-loader/url'},{loader:'file-loader'}]
            }
        ]
    },
    plugins: [
        new HTMLWebpackPlugin({
          title: 'css'
        })
    ]
};

通过useable控制样式显示

// webpack.config.js
const path = require('path');
const HTMLWebpackPlugin = require('html-webpack-plugin');
const webpack = require('webpack');

module.exports = {
    entry: {
        index: './src/index.js',
    },
    output: {
        path: path.resolve(__dirname, 'dist'),
        // publicPath: './dist/',
        filename: '[name].bundle.js',
    },
    module: {
        rules: [
            {
                test: /\.css$/,
                use: [{loader:'style-loader/useable'},{loader:'css-loader'}]
            }
        ]
    },
    plugins: [
        new HTMLWebpackPlugin({
          title: 'css'
        })
    ]
};
// index.js
import base from './css/base.css';
import common from './css/common.css';

base.unuse();
base.use();

css tree-shaking, 处理无用代码

$_> npm i -D purifycss-webpack purify-css glob-all
// webpack.config.js
const path = require('path');
const HTMLWebpackPlugin = require('html-webpack-plugin');
const webpack = require('webpack');
const glob = require('glob-all');
const MiniCssExtractPlugin = require("mini-css-extract-plugin");
const PurifyCSSPlugin = require('purifycss-webpack');

module.exports = {
    entry: {
        index: './src/index.js',
    },
    output: {
        path: path.resolve(__dirname, 'dist'),
        filename: '[name].bundle.js',
    },
    module: {
        rules: [
            {
                test: /\.css$/,
                use: [{ loader: MiniCssExtractPlugin.loader },
                { loader: 'css-loader' },
                ]

            }
        ]
    },
    plugins: [
        new HTMLWebpackPlugin({
            title: 'css'
        }),
        new MiniCssExtractPlugin({
            filename: "[name].css",
            chunkFilename: "[id].css"
        }),
        new PurifyCSSPlugin({
            paths: glob.sync([path.join(__dirname, './index.html'),path.join(__dirname, './src/index.js')]),
        }) // 需要处理的文件
    ],
    mode: 'development'
};
// base.css
body {
    display: flex;
    background: orange;
}
.box {  
    color: #000;
}
.box2 { // 这个类名会被删除
 color: #fff;
}
// index.js
import './css/base.css';
let div = document.createElement('div');
div.classList.add('box');
document.body.appendChild(div);

压缩css

$_>  npm i -D  optimize-css-assets-webpack-plugin cssnano

const path = require('path');
const CleanWebpackPlugin = require('clean-webpack-plugin');
const HtmlWebpackPlugin = require('html-webpack-plugin');
const MiniCssExtractPlugin = require("mini-css-extract-plugin");
const optimizeCss = require('optimize-css-assets-webpack-plugin');

module.exports = {
    entry: './src/scripts/index.js',
    output: {
        filename: 'scripts/[name].js',
        path: path.resolve(__dirname, 'dist')
    },
    module: {
        rules: [
            {
                test: /\.css$/,
                use: [{ loader: MiniCssExtractPlugin.loader },
                { loader: 'css-loader' },
                ]
            },
        ]
    },
    plugins: [
        new CleanWebpackPlugin('/dist/'),
        new HtmlWebpackPlugin({
            filename: 'index.html',
            template: './src/index.html',
            minify: { // 压缩
                collapseInlineTagWhitespace: true,
                collapseWhitespace: true
            },
        }),
        new MiniCssExtractPlugin({
            filename: "css/[name].css",
            chunkFilename: "[id].css",
        }),
        new optimizeCss({ // 压缩css
            assetNameRegExp: /\.css$/g,
            cssProcessor: require('cssnano'),
            cssProcessorOptions: { discardComments: { removeAll: true } },
            canPrint: true
        }),
    ]
};