There is definitely some misunderstanding.

We are talking about _three_ models:

1. That of WA model, which operates "animation values" (sections 4.1-4.2 of the spec)
2. Used values (CSS used values or SVG elements attributes values)
3. Rendered values (what are really presented on user screen)

Since rendered values are not exposed in JS in any form, there is no sense to talk about them at all. But the spec does, saying, that actual rendering must not reflects changes immediately. That's very strange statement, since (a) it is simply out of scope of the spec, (b) it contradicts common practice of setting/getting certain values (width, margin, etc) to make browser flush changes and do actual rendering.

The relation between 1) and 2) is much more interesting. The spec says:
<blockquote>Querying the computed style of a property affected by animation, or the value of an attribute affected by animation, returns the value based the current state of the model</blockquote>
I take it as "animation values and used values must be are the same", but spec simply provide no method to test that equality. *You can't access animation values*. There is no direct method to "change width in web animation model". Web Animation provide timings, durations, rates, but not actual time fractions and computed values.