ash/webui/diagnostics_ui/resources/touchscreen_tester.html

<style>
  #canvas-dialog {
    --cr-dialog-width: 100%;
    --cr-dialog-height: 100%;
  }

  #canvas-dialog [slot='body'] {
    overflow: hidden;
    padding: 0;
  }
</style>

<!-- TODO(wenyu): Strings need translation. -->
<cr-dialog id="intro-dialog">
  <div slot="body">
    Test your touchscreen
    <div>
      Test your touchscreen by tapping or swiping the screen with up to 5
      fingers. Wipe down your screen with a microfiber cloth for a more
      accurate test.
    </div>
    <cr-button on-click="onStartClick">Get started</cr-button>
  </div>
</cr-dialog>

<cr-dialog id="canvas-dialog">
  <div slot="body">
    <canvas id="canvas"></canvas>
  </div>
</cr-dialog>