ui/webui/resources/cr_elements/hidden_style_css.html

<link rel="import" href="../html/polymer.html">

<!-- Common style for Material Design WebUI, such that the |hidden| attributes
  works within Shadow DOM. -->
<dom-module id="cr-hidden-style">
  <template>
    <style>
      /* Included here so we don't have to include "iron-positioning" in every
       * stylesheet. See crbug.com/498405. */
      [hidden] {
        display: none !important;
      }
    </style>
  </template>
</dom-module>