[css-transforms] Why doesn't perspective-origin just use <position>?

The 'perspective-origin' property specifies a point in a 2d box.  Why
isn't its grammar just using the <position> production?  It looks like
it's reusing the 'transform-origin' grammar, but without the 3rd
coordinate.

<position> is complicated enough that it's annoying to parse a grammar
near it to figure out what's different/excluded.

~TJ