Level 16 is a time eater again. It gives the first clue in the source code itself like this one. But, what after that?
UNAVAILABLE <!-- level 17: /images" -->
Just type images as well in the url and see yourself.
You will find something like this in the next source. Another of those background images. But this image isn’t actually an image this time. Try opening it in a text editor, say notepad, and you will get to level 17 finally.
Level 17 is here: /unavailable/Ducky.php.