title: got error when run ng build --prod --base-href in angular 4
body: <p>Here is the error message when running <code>ng build --prod --base-href</code></p>
  
  <blockquote>
    <p>ERROR in ./node_modules/bootstrap/dist/css/bootstrap.min.css
        Module build failed: BrowserslistError: Unknown browser major
            at error (C:\Users\Gurpreet_Singh29\Practice Work\SOPID\SOP\node_modules\postcss-merge-rules\node_modules\browserslist\index.js:37:11)
            at Function.browserslist.checkName (C:\Users\Gurpreet_Singh29\Practice
    Work\SOPID\SOP\node_modules\postcss-merge-rules\node_modules\browserslist\index.js:320:18)
            at Function.select (C:\Users\Gurpreet_Singh29\Practice Work\SOPID\SOP\node_modules\postcss-merge-rules\node_modules\browserslist\index.js:438:37)
            at C:\Users\Gurpreet_Singh29\Practice Work\SOPID\SOP\node_modules\postcss-merge-rules\node_modules\browserslist\index.js:207:41
            at Array.forEach ()
            at browserslist (C:\Users\Gurpreet_Singh29\Practice Work\SOPID\SOP\node_modules\postcss-merge-rules\node_modules\browserslist\index.js:196:13)
            at C:\Users\Gurpreet_Singh29\Practice Work\SOPID\SOP\node_modules\postcss-merge-rules\dist\index.js:261:51
            ...</p>
  </blockquote>
  
  <p>How do I resolve this?</p>