chromium/ash/webui/common/resources/cr_elements/cr_view_manager/cr_view_manager.html

<style>
  :host ::slotted([slot=view]) {
    bottom: 0;
    display: none;
    left: 0;
    position: absolute;
    right: 0;
    top: 0;
  }

  :host ::slotted(.active),
  :host ::slotted(.closing) {
    display: block;
  }
</style>
<slot name="view"></slot>