Whenever the test requires a CSS stylesheet, we could use a generated data URL. Also, a number of tests do use the data URL.
What we should do is to create a utility to convert String to data URL.

public static String toDataUrl(String css) {
  return "data:base64," + Base64.getUrlEncoder().encodeToString(css.getBytes(StandardCharsets.UTF_8));
}

(Came out of a discussion in https://github.com/openjdk/jfx/pull/1394)