• 用vue-cli3搭建创建的项目，创建时选择的preset为vue-router,vuex,sass,babel,es
• vue.config.js配置如下

module.exports = {
  outputDir: 'dist/umall',
  publicPath: './',
  runtimeCompiler: true,
  productionSourceMap: false,
  css: {
    modules: true,
    sourceMap: false,
    loaderOptions: {
      sass: {
        data: `@import "@/assets/reset.scss";`
      },
      postcss: {
        plugins: [
          require('postcss-import'),
          require('postcss-px-to-viewport')({
            'viewportWidth': 375, // (Number) The width of the viewport.
            'viewportHeight': 667, // (Number) The height of the viewport.
            'unitPrecision': 3, // (Number) The decimal numbers to allow the REM units to grow to.
            'viewportUnit': 'vw', // (String) Expected units.
            'selectorBlackList': ['.ignore', '.hairlines'], // (Array) The selectors to ignore and leave as px.
            'minPixelValue': 1, // (Number) Set the minimum pixel value to replace.
            'mediaQuery': false // (Boolean) Allow px to be converted in media queries.
          })
        ]
      }
    }
  },
  devServer: {
    open: true,
    disableHostCheck: true
  },
  parallel: require('os').cpus().length > 1
}

• 引入了ui库[vant]（https://youzan.github.io/vant...）

出现的问题： 无法引入vant-ui的css文件，如果将vue.config.js中的css配置项注释掉，就可以引入vant-ui的css了，不懂vue-cli是如何处理的？