title: Preact class attribute is empty
body: <p>Today I am trying to use CSS in preact with webpack.
  It is almost working for me.</p>
  
  <p>This is target code. In browser class attribute is empty. I tried className with same result, but CSS code is included in bundle.</p>
  
  <pre><code>import css from './todo-list.css';
  
  export class C extends Component {
    render() {
      return &lt;h1 class={css.bordered}&gt;title&lt;/h1&gt;;
    }
  }
  
  
  </code></pre>
  
  <p>If I just import css and don't reference it anywhere from the code
  then CSS code is skipped.</p>
  
  <p>So as for now my workaround is to reference css somewhere and hardcode CSS classes as string literals.</p>
  
  <pre><code>import css from './styles.css';
  
  export class C extends Component {
    render() {
      return &lt;h1 class='bordered'&gt;&lt;span class={css}&gt;title&lt;/span&gt;&lt;/h1&gt;;
    }
  }
  
  
  </code></pre>
  
  <p>I wasn't able to spot any issue with className for preact. It looks like it supported out of the box without.
  Any ideas about debugging class attribute are appreciated. </p>
  
  <p>Versions: </p>
  
  <pre><code>    "css-loader": "^2.1.1",
      "preact": "^8.4.2",
      "webpack": "^4.28.1"
  
  </code></pre>
  
  <p>Loader configuration:</p>
  
  <pre><code>      {
          test: /\.css$/i,
          use: ['style-loader', 'css-loader']
        },
  
  </code></pre>
  
  <p>In browser css is a function and it returns empty object:</p>
  
  <pre><code>_todo_list_css__WEBPACK_IMPORTED_MODULE_5___default
  ƒ getModuleExports() { return module; }
  </code></pre>