force_input_autocomplete.user.js

// ==UserScript==
// @name            Force Autocomplete
// @description     enable autocomplete for all HTML input fields
// @author          disrupted
// @version         0.1
// @match           http://*/*
// @grant           none
// ==/UserScript==

var initialDelay = 2000; // adjust to make it work with all websites
// =================================================================

(function() {
  'use strict';
  var counter = 0;
  window.addEventListener('load', function() {
    setTimeout(function() {
      var forms = document.getElementsByTagName('form');
      for (var i = 0; i < forms.length; i++) {
        forms[i].autocomplete = 'on';
      }
      var inputfields = document.getElementsByTagName('input');
      for (var j = 0; j < inputfields.length; j++) {
        if (inputfields[j].getAttribute('autocomplete') == 'off') {
          inputfields[j].autocomplete = 'on';
          var oldPlaceholder = inputfields[j].getAttribute('placeholder');
          inputfields[j].placeholder = oldPlaceholder + ' + autocomplete';
          counter++;
        }
      }
      if (counter !== 0) {
        console.log('[Userjs] forcing autocomplete for ' + counter + ' input fields.');
      }
    }, initialDelay);
  }, false);
})();