How to unset image-rendering for Safari 11+ without yielding "property stroke-color doesn't exist" on W3C CSS Validator

Question

To avoid blurry downscaled images in Chrome, I followed GoojajiGreg's answer to use media queries to set image-rendering to -webkit-optimize-contrast on Chrome 29+ (which reduces the blurriness in Chrome) and to unset it on Safari 11+ (to avoid pixelated images in Safari). The CSS code from the answer is reproduced below.

@media screen and (-webkit-min-device-pixel-ratio:0) and (min-resolution:.001dpcm) {
  img {
    image-rendering: -webkit-optimize-contrast !important;
  }
}

/* Unset for Safari 11+ */
@media not all and (min-resolution:.001dpcm) {
  @supports (-webkit-appearance:none) and (stroke-color:transparent) {
    img {
      image-rendering: unset !important;
    }
  }
}

This code works for me, but I noticed that when I enter it into the W3C CSS Validator, the validator yields the following error.

Property stroke-color doesn't exist : transparent

Is there a way to improve the code above so that it does not yield this error?

I see that a CSS property with a similar name, -webkit-text-stroke-color, does exist, and when I replace stroke-color with -webkit-text-stroke-color, no error is found and the code "validates as CSS level 3 + SVG". However, on the -webkit-text-stroke-color MDN Web Docs page, I see the following warning, which suggests this may not be a preferred solution.

Non-standard: This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.