ui/webui/resources/cr_elements/cr_a11y_announcer/cr_a11y_announcer.html

<style>
  :host {
    clip: rect(0 0 0 0);
    height: 1px;
    overflow: hidden;
    position: absolute;
    width: 1px;
  }
</style>

<div id="messages" role="alert" aria-live="polite"></div>