function relative(rootURL, targetURL) {
  const result = relativeIfPossible(rootURL, targetURL);

  return typeof result === "string" ? result : normalize(targetURL);
}
Make a path relative to a URL or another path. If returning a
relative URL is not possible, the original target will be returned.
All returned URLs will be normalized.

@param aRoot The root path or URL.
@param aPath The path or URL to be made relative to aRoot.
@return A rootURL-relative (if possible), normalized URL value.