2.1.1628 Part 1 Section 126.96.36.199, aln (Alignment)
a. The standard states that aln is not used when applied to a box which is not an operator emulator.
Office does not allow aln to be applied to a box which is not an operator emulator.
b. The standard says aln applies only to the box function even though the schema allows it as a child of math runs.
In Word, the aln element applies to the box function and to math runs.