How to add HTML5 attribute "autocomplete" to form fields?
I think it's possible to add this attribute to all input fields using a bit of JS, something like this should work:

(function($, Drupal) {
  Drupal.behaviors.autocompleteFieldBehavior = {
    attach: function (context) {
      $(context).find('input').each(function () {
        var name = $(this).attr('name');
        if ($(this).attr('autocomplete').length === 0) {
          if (name) {
            $(this).attr('autocomplete', name);
          }
          else {
            $(this).attr('autocomplete', 'Default autocomplete value.');
          }
        }
      });
    }
  };
})(jQuery, Drupal);


The other solution would be alter all forms using hook_form_alter, see @berramou's comment.