Add a template or format attribute for settings that have multiple value parts such as background-position so that I could specify an animation attribute such as:

{'background-position' : {
from: 0,
to: 100,
template: 'center {value}px'
}}

and the 'tween' values will be added to the template. Alternatively, the template or format attribute might receive a reference to a function that would receive the value and would return the value
to be set.

I guess that even a 'prefix' attribute would do:

{'background-position' : {
from: 0,
to: 100,
unit: 'px',
prefix: 'center '
}}

and it would probably far easier to add.

However, the template: {function reference} would provide the most flexibility.