2.1.135 Part 1 Section 17.4.24, insideH (Table Cell Inside Horizontal Edges Border)
For additional notes that apply to this portion of the standard, please see the notes for bar, §17.3.1.4(a-c) ; bdr, §17.3.2.4(a) ; left, §17.15.2.21(a) .
a. The standard states that if the table has cell spacing then inside horizontal borders are displayed.
In Word, when a table has cell spacing, the appearance of the insideH border is still subject to the conflict resolution algorithm defined by the tcBorders and tblBorders elements.