chrome/browser/resources/chromeos/parent_access/parent_access_ui.html

<style>
  html,
  body {
    height: 100%;
  }

  #webviewDiv {
    background-color: var(--cros-bg-color-elevation-1);
    height: 100vh;
  }

  #webview {
    display: flex;
    height: 100%;
  }

  #webview[hidden],
  .spinner-container[hidden] {
    display: none;
  }

  paper-spinner-lite {
    display: none;
    height: 38px;
    margin-bottom: 28px;
    width: 38px;
  }

  paper-spinner-lite[active] {
    display: inline-block;
  }

  .spinner-container {
    align-items: center;
    display: flex;
    flex-direction: column;
    height: 100%;
    justify-content: center;
    width: 100%;
  }
</style>
</div>
<div id="webviewDiv" role="dialog"
    aria-label="$i18n{pageTitle}">    
  <webview id="webview" hidden$="[[webviewLoading]]"></webview>
  <div class="spinner-container" hidden$="[[!webviewLoading]]">
    <paper-spinner-lite class="spinner" active="[[webviewLoading]]">
    </paper-spinner-lite>
    <div aria-live="polite" aria-label$="$i18n{webviewLoadingMessage}">
      $i18n{webviewLoadingMessage}
    </div>
  </div>
</div>