Accept string for keyboardEventTarget option

src/objectliterals.jsdoc

@@ -30,7 +30,8 @@
  *     then it gets set by using `window.devicePixelRatio`.
  * @property {ol.Collection|Array.<ol.interaction.Interaction>|undefined} interactions
  *     Interactions that are initially added to the map.
- * @property {Element|Document|undefined} keyboardEventTarget The element to listen to keyboard events on.
+ * @property {Element|Document|string|undefined} keyboardEventTarget
+ *     The element to listen to keyboard events on.
  *     This determines when the `KeyboardPan` and `KeyboardZoom` interactions trigger. For example, if
  *     this option is set to `document` the keyboard interactions will always trigger. If this option
  *     is not specified, the element the library listens to keyboard events on is the map target (i.e.

src/ol/map.js

@@ -1336,8 +1336,14 @@ ol.Map.createOptionsInternal = function(options) {
   /**
    * @type {Element|Document}
    */
-  var keyboardEventTarget = goog.isDef(options.keyboardEventTarget) ?
-      options.keyboardEventTarget : null;
+  var keyboardEventTarget = null;
+  if (goog.isDef(options.keyboardEventTarget)) {
+    // cannot use goog.dom.getElement because its argument cannot be
+    // of type Document
+    keyboardEventTarget = goog.isString(options.keyboardEventTarget) ?
+        document.getElementById(options.keyboardEventTarget) :
+        options.keyboardEventTarget;
+  }
 
   /**
    * @type {Object.<string, *>}