IntelliJ JCEF browser does not load CSS images such as url("ul.png")

I am using the JCEF browser as part of the Asciidoc-Editor.
(There is an option to use Swing instead but the results which jbcef are much better.)

Within some file.adoc I set the following attribute:

:stylesheet: style.css

And within the style.css I use some image, e. g.:

ul {
  padding-left: 0.36cm;
  list-style: square outside url("ul.png");
}

When I generate the HTML it looks fine.

Within the preview, I got sqares instead of the given image. The type of image file does not matter, besides PNG i've also tried GIF or SVG. The developer tools report a file not found error for the image file.

If I change the url to a global path on the local file system, it works fine.

I also tried to place some SVG directly into the url, as described here: https://css-tricks.com/lodge/svg/09-svg-data-uris/. The developer tools report a security error for the image file because chromium does only accept data from the local file system due to security settings.