title: The opacity value was changed to 1% after building the Reacjs project
body: <p>Im building a Reactjs project and using scss instead of css.
  In the App.scss file, I set opacity: 87%; (note: unit is percent).</p>
  
  <p>Everything worked normally (the opacity value still equal 87%) when I run command: yarn start</p>
  
  <p>But when I run command: yarn build and check value opacity in the file build/static/css/main.86352307.chunk.css then the opacity value was changed to 1%.</p>
  
  <p>File App.scss:</p>
  
  <pre><code>.App {
    text-align: center;
    opacity: 87%;
  }
  </code></pre>
  
  <p>After build project: File build/static/css/main.86352307.chunk.css</p>
  
  <pre><code>.App{text-align:center;opacity:1%}
  /*# sourceMappingURL=main.86352307.chunk.css.map */
  </code></pre>
  
  <p>Step to reproduce issue</p>
  
  <pre><code>#1. npx create-react-app my-app
  #2. cd my-app
  #3. yarn add node-sass
  #4. rename file App.css to App.scss and then add opacity: 87% in class .App
  
  #Testcase 1: run yarn start =&gt; the opacity value (87%) is apply for class .App
  #Testcase 2: run yarn build =&gt; the opacity value (1%) is apply for class .App
  </code></pre>
  
  <p>Please tell me know the reason.</p>