This package provides an opportunity to modify @angular/cli project's webpack configuration without "ejecting".

For angular 6:

$ npx -p @angular/cli ng new my-project && cd my-project # create new Angular CLI project
$ npm i -D ngw # installing an improved cli-eject
$ ./node_modules/.bin/ngw --set-up # run via terminal in project root
Set up went successfully!

For angular 5 use npm i -D ngw@angular5

Last command installation (ngw --set-up) makes two things:

1. Changes scripts in package.json that starts from ng to ngw
2. Creates file ngw.config.ts in project root where you can redefine webpack.Configuration used by @angular/cli

So just make changes to the webpack config in appeared ngw.config.ts

You may like to do npm i -D @types/webpack for better experience.

This little piece of code in your ngw.config removes unused selectors from your CSS:

const PurifyCSSPlugin = require('purifycss-webpack');
const path = require('path');
const glob = require('glob');

export default function(config) {
    config.plugins.push(
      new PurifyCSSPlugin({
        paths: glob.sync(path.join(__dirname, '**/*.html'))
      })
    );
    return config;
}

export default function(config, options) {
  //common config modification
  ...
  switch(options.buildOptions.enviroment) {
    case 'prod':
      config = productionModificationsMerged(config);
      break
    case 'dev':
      //etc
  }
}

For complex cases it's more appropriate to use ng eject command. Default building process could be changed significanlty in further @angular/cli releases so your customization could break (or became broken).