2.1.620 Part 1 Section, firstHeader (First Page Header)

a.   The standard does not place any restrictions on the length of the value for this element.

Office requires that the length of the value for this element be less than or equal to 255.

b.   The standard makes no mention of an xml:space attribute.

Excel uses this attribute to specify, using the W3C space preservation rules, how white space should be handled for the element's contents; its allowed values are defined by the XML 1.0 specification.