--- a/testing/marionette/element.js
+++ b/testing/marionette/element.js
@@ -809,16 +809,19 @@ element.inViewport = function(el, x = un
  * @param {Window} window
  *     Window object.
  * @param {number=} x
  *     Horizontal offset relative to target.  Defaults to the centre of
  *     the target's bounding box.
  * @param {number=} y
  *     Vertical offset relative to target.  Defaults to the centre of
  *     the target's bounding box.
+ *
+ * @return {boolean}
+ *     True if visible, false otherwise.
  */
 element.isVisible = function(el, x = undefined, y = undefined) {
   let win = el.ownerDocument.defaultView;
 
   // Bug 1094246: webdriver's isShown doesn't work with content xul
   if (!element.isXULElement(el) && !atom.isElementDisplayed(el, win)) {
     return false;
   }