2.1.173 Part 4 Section 2.4.70, tl2br (Table Cell Top Left to Bottom Right Diagonal Border)
a. The standard places no restrictions on the border style used in the val attribute.
Word displays diagonal cell borders as a single solid line, except borders with dotted, dashed, dashSmallGap, dotDash, dotDotDash, and double border styles.
b. The standard states that if this element is omitted then the table's border settings may be used.
In Word, if this element is omitted, then the table styles border is used.