2.1.652 Part 4 Section 18.104.22.168, formula (Formula)
a. The standard states that the content of this element is a formula whose calculated value specifies the criteria for the conditional formatting rule.
If the parent element for formula is rdn ("[ECMA-376] Part 4 §22.214.171.124; rdn"), then the formula element represents a formula for a defined name and is defined by the grammar in Name Formulas (§126.96.36.199.4, Name Formulas).
If the parent element for formula is cfRule ("[ECMA-376] Part 4 §188.8.131.52; cfRule"), then the formula element represents a formula for a conditional formatting rule, and is defined by the grammar in Conditional Formatting Formulas (§184.108.40.206.2, Conditional Formatting Formulas).
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.