2.1.526 Part 1 Section 17.18.2, ST_Border (Border Styles)
a. The standard states that the thinThickLargeGap, thinThickMediumGap, and thinThickSmallGap border styles specify line borders consisting of a thin line contained within a thick line contained within a thick thin.
Word uses these styles to specify line borders consisting of a thin line contained within a thick line.
b. The standard places no restrictions on when the inset and outset border styles should be honored.
Word displays inset and outset borders using the single border style when the border is not a table border or when the resolved value of the tblCellSpacing element for a row is nil.
c. The standard states that the inset and outset border styles specify a line border consisting of an inset/outset set of lines around the parent object.
Word displays these border styles as defined in the CSS2 specification [CSS-LEVEL2].
d. The standard states that custom is a valid enumeration value.
In Word, if the enumeration value of an ST_Border simple type is custom, then the file is considered corrupt.