Diagonal lines are not supported in table cells, in contrast with MS Word, which supports diagonal lines.
Steps to Reproduce:
User Profile Reset: No
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:53.0) Gecko/20100101 Firefox/53.0
You have submitted the same request already as bug 88807.
*** This bug has been marked as a duplicate of bug 51665 ***