css/css-values/urls/support/fragment-only-urls.css

:root {
    --fragment-image-url: url("#foo");
    --fragment-cursor-url: url("#foo"), pointer;
}

#external-unquoted {
    background-image: url(#foo);
    cursor: url(#foo), pointer;
}

#external-quoted {
    background-image: url("#foo");
    cursor: url("#foo"), pointer;
}

#external-variable {
    background-image: var(--fragment-image-url);
    cursor: var(--fragment-cursor-url);
}