[css3-transitions] interpolation between value types

While http://dev.w3.org/csswg/css3-transitions/#animation-of-property-types- describes how each property type undergoes a transition, it doesn't define how interpolation should occur between value types. eg. between a percentage and a length.

I would imagine that you'd compute a length from the percentage, then interpolate over the lengths. But this is a little more complicated in the case of background-position, for instance. If you have:

{
background-image: url('1.gif'), url('2.gif'), url('3.gif');
                background-size: 100px 100px, 200px 200px, 300px 300px;
                background-position: 50%;
                transition: background-position 5s;
                width: 500px;
                height: 500px;
}


then set element.style.backgroundPosition="100px";


What does a transition and its corresponding computed value look like in this case?