I understand your reasoning on the webkit prefix stuff being tangentially related to epub3 via the systems its being rendered on, but I myself would not feel comfortable including it in an actual, for-distribution, epub's css. Only because my reading of the latest epub spec indicates that the only css prefix still a part of the spec is -epub. I'm not judging though. You do you.
https://www.w3.org/TR/epub-33/#sec-css-prefixed