First of all, I appreciate that you’re thinking about a fallback at all. Most people just implement technologies without thinking any further.
Then, one thing I’m always doing right in the beginning is to add a class to the body (or html root element) using JS which serves as my hook to apply specific styles that are only seen if JS is enabled. The reverse approach (which is used by some people, too) would be to add a “no JS” class to the html and
remove it with JS then. This way you can do the following with your box: If JS is not available/enabled then make it a simple CSS hover function.
Code:
#inkassobox:hover {height: 280px;}