2.1.1874 Part 4 Section 22.214.171.124, transp (Transparent (Phantom))
a. The standard says if transparency is off, then the contents of the phantom are ignored during layout.
In Word, if transparency is off, special spacing is not added before and after the phantom, but the contents are not ignored during layout.
b. The standard allows transparency to be set on a phantom with any contents.
In Word, transparency does not work as specified unless the phantom contains another math function and does not start or end with a math operator.