Form placeholder text

Is there a way/snippet to enable the use of placeholder text in the lwa-username-input field of the legacy-default form? We recently stopped allowing usernames and now only allow email addresses so would like an even more obvious placeholder to prompt the user to only input an email.

I tried adding it to the plugin code as an experiment but it’s not showing up:

  <td class="lwa-username-input">
                        <input type="text" name="log" placeholder="Enter your email address" />
                    </td>

Also, is there an error message that could say, “Please use an email address” when usernames are disallowed but a user attempts to use a username to login? It would save lots of emails when our users try a username and get a generic error. Thank you!