In Drupal 7, some forms (contact, comment, register) store information of the submitting anonymous user in cookies like Drupal.visitor.* for pre-populating forms later with that information if the form included the CSS class 'user-info-from-cookie'.
In Drupal 8, that information is stored on the browser local storage, and instead of using a CSS class in the form for opting in to this functionality, now the attribute data-user-info-from-browser must be used.
This functionality was included in Drupal 7 in the JS behavior Drupal.behaviors.fillUserInfoFromCookie, which has been renamed in Drupal 8 to Drupal.behaviors.fillUserInfoFromBrowser.
This pre-populating feature only works for input that have one of the following name : 'name', 'mail' or 'homepage'.
For using this feature in your forms with Drupal 7 you needed:
$form['#attributes']['class'][] = 'user-info-from-cookie';
In Drupal 8 it will become:
$form['#attached']['library'][] = 'core/drupal.form';
$form['#attributes']['data-user-info-from-browser'] = TRUE;
