UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/59.0.3071.115 Safari/537.36

Steps to reproduce the problem:
1. Enter this to url bar: `data:text/html,<div id="foo" style="grid-template-rows: [a] repeat(2, [b] 20px)"></div><script>alert(foo.style.gridTemplateRows);</script>`

What is the expected behavior?
Should print "[a b] 20px [b] 20px".

What went wrong?
It prints "[a] [b] 20px [b] 20px" instead.

Did this work before? N/A 

Does this work in other browsers? N/A

Chrome version: 59.0.3071.115  Channel: stable
OS Version: OS X 10.12.5
Flash Version: 

"[a] [b] 20px [b] 20px" is not a valid <track-list> value and it's not roundtripping with that value. `[a] [b]` should merge instead.
I came across this when I was implementing this in servo. Also there is more fundamental bug filed[1] for repeat function which can make this obsolete. But computed value of this repeat function can be properly serialize. The problem is just about specified value. I think it is more straightforward to fix this first.

[1] https://bugs.chromium.org/p/chromium/issues/detail?id=716114