Webpack SASS Autoprefixer?
Я делал как-то так.Настройка Autoprefixer под Webpack
module: {
    rules: [
      {
        test: /\.sass|scss$/,
        use: ExtractTextPlugin.extract({
          fallback: 'style-loader',
          use: ['css-loader', 'sass-loader'],
          publicPath: './dist/assets/css/'
        })
      },
{
test: /\.css/,
loader: 'css!style!autoprefixer!?browsers=last 12 versions',
},
      {
        test: /\.js$/,
        exclude: /(node_modules|bower_components)/,
        loader: 'babel-loader',
        query: {
          presets: ['env'],
          plugins: ['transform-runtime']
        }
      }
    ]
доки для этого
https://github.com/passy/autoprefixer-loader