2.1.727 Part 4 Section 3.8.10, cellXfs (Cell Formats)
a. The standard states that the maxOccurs value for the xf child element is unbounded.
Excel limits the occurrences of this element to 65430.
b. The standard states that both the cell style xf records and cell xf records must be read to understand the full set of formatting applied to a cell.
In Office, only the cell xf record defines the formatting applied to a cell.
c. The standard does not limit the number of xf elements inside cellXfs and cellStyleXfs, or note any relationship between them.
Office requires that the combined quantity of xf records inside cellXfs and non-built-in xf records inside CellStyleXfs must not exceed 65430 elements.