XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (312 page)

Expression	Syntax
IfExpr	if ( Expr ) then ExprSingle else ExprSingle

The
else
branch is constrained to be an
ExprSingle
(an expression containing no top-level comma) because a trailing comma would be ambiguous when the expression appears, for example, as an argument in a function call. The
then
branch is constrained to be an
ExprSingle
purely for symmetry. Any expression can be used as the condition, and although it would be unusual for this expression to use the
,
operator, there is no reason to disallow it.