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Unicode non-breaking space ( ) is a whitespace character that occurs quite often in HTML documents. So I think it should be handled by Rendering text section. Unicode defines other whitespace characters (http://en.wikipedia.org/wiki/Whitespace_(computers)#Unicode) too. I think they should be handled correctly as whitespace by the spec.
*** Bug 25306 has been marked as a duplicate of this bug. ***
This was fixed in https://github.com/w3c/webdriver/commit/eddab00e1a55045457f2651bdc52469648895f05