[css-round-display] Extend 2D rotation transform function for polar coordinates

In the previous discussion about 'polar-orientation' property in CC[1] and
F2F meeting at TPAC [2], 
the effects of the property are decided to be achieved by the rotate
property with additional keywords.

Therefore, I suggest extended 2d rotation transform function for polar
coordinates [3].
The 2d rotation function 'rotate()' used in polar coordinates is specified
as follows:

	rotate() = rotate( <angle> | center | counter-center)

Values used for the function indicate as follows: 

* center
   Rotate an element by the polar-angle value. Therefore, when the anchor
point of an element is center point of the element and there is a
perpendicular line which passes through the anchor point, the line meets the
origin of the containing block.

* counter-center
   Rotate an element by the angle of the polar-angle plus 180 degrees.

I think the extended rotate function is useful for positioning elements in
polar coordinates.
There are several usecases which show elements aligning toward the center
point of round display [4].

Are names of keywords appropriate?
Is definition of extended 2d rotation transform function reasonable?


Thanks,
Jihye Hong

[1] https://lists.w3.org/Archives/Public/www-style/2015Oct/0199.html 
[2] http://log.csswg.org/irc.w3.org/css/2015-10-26/#e614073
[3]
https://drafts.csswg.org/css-round-display/#2d-rotation-transform-function
[4] http://anawhj.github.io/jRound/demo/facebook/circle.html