transform-origin: [ center | left | right | top | bottom | percent | length ] {1..2}
50% 50%
No
If two terms are given, then usually the first term must give the horizontal component, and the second must give the vertical.
(The only exception is that if both terms are given as keywords, then the order may be swapped: so center left and left center and 0% center and left 50% are all legal and equivalent to each other, whereas 50% left is not a legal value.)
If only one term is given, then the second component is assumed to be center. (In particular, if only a percentage or length is given, then it is assumed to be the horizontal coordinate.)