With Webpack, is it possible to generate CSS only, excluding the output.js?

Unfortunately, that is currently not possible by design. webpack started as a JavaScript bundler which is capable of handling other "web modules", such as CSS and HTML. JavaScript is chosen as base language, because it can host all the other languages simply as strings. The extract-text-webpack-plugin is just extracting these strings as standalone file (thus the name).

You're probably better off with PostCSS which provides various plugins to post-process CSS effectively.

There is an easy way, no extra tool is required.

There is an easy way and you don't need extra libraries except which you are already using: webpack with the extract-text-webpack-plugin.

In short:

Make the output js and css file have identical name, then the css file will override js file.

A real example (webpack 2.x):

import path from 'path'
import ExtractTextPlugin from 'extract-text-webpack-plugin'

const config = {
  target: 'web',
  entry: {
    'one': './src/one.css',
    'two': './src/two.css'
  },
  output: {
    path: path.join(__dirname, './dist/'),
    filename: '[name].css' // output js file name is identical to css file name
  },
  module: {
    rules: [
      {
        test: /\.css$/,
        use: ExtractTextPlugin.extract({
          fallback: 'style-loader',
          use: 'css-loader'
        })
      }
    ]
  },
  plugins: [
    new ExtractTextPlugin('[name].css') // css file will override generated js file
  ]
}