This is definitely an epubcheck 3.01. bug. Apparently it doesn't like rgb color-stop values. Try replacing them with a hex value. The following appears to work:
Code:
background-image: -webkit-gradient(radial,50% 50%, 0, 50% 50%, 26,from(white),color-stop(0.4, #F8FF80),to(transparent));
At least it causes no validation errors.