UserAgent: Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.84 Safari/537.36

Example URL:
http://codepen.io/levisan/pen/MeyMVp

Steps to reproduce the problem:
1. Go to http://codepen.io/levisan/pen/MeyMVp
2. Hover over the <div>
3. The CSS transition does not work.

What is the expected behavior?

What went wrong?
The CSS transition should ease between the two set background sizes.

4. Uncomment lines 11 and 18 of the CSS, so that background-size is set as both <height> and <width> and not just a single value.
5. Now the transition works

Does it occur on multiple sites: Yes

Is it a problem with a plugin? No 

Did this work before? Yes Approximately 14 days ago

Does this work in other browsers? Yes 

Chrome version: 51.0.2704.84  Channel: stable
OS Version: 10.0
Flash Version: Shockwave Flash 21.0 r0

Still works in current versions of IE, Edge, and Firefox.