title: webpack convert css unit with uglifyjs plugin
body: <p>It's weird, actually, I think it is quite easy to come across, but I have found nothing about this issue~</p>
  
  <p>I have tested with / without uglifyjs plugin, and I'm quite sure that this plugin results in this issue.</p>
  
  <p>In my style files, only px / em have been used, but after compression, some of px has been converted to pc / pt, totally have no clue~</p>
  
  <p><a href="https://i.stack.imgur.com/GspUp.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/GspUp.png" alt="enter image description here"></a></p>