--- a/testing/marionette/element.js
+++ b/testing/marionette/element.js
@@ -770,24 +770,27 @@ element.coordinates = function(node, x =
     x: box.left + x,
     y: box.top + y,
   };
 };
 
 /**
  * This function returns true if the node is in the viewport.
  *
- * @param {Element} element
+ * @param {Element} el
  *     Target element.
  * @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 if |el| is in viewport, false otherwise.
  */
 element.inViewport = function(el, x = undefined, y = undefined) {
   let win = el.ownerDocument.defaultView;
   let c = element.coordinates(el, x, y);
   let vp = {
     top: win.pageYOffset,
     left: win.pageXOffset,
     bottom: (win.pageYOffset + win.innerHeight),