2.1.669 Part 1 Section 18.4.12, t (Text)
a. 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.
b. The standard places no restrictions on the length of the value of the t element.
Excel restricts the value of this element to 32767 characters.