Run the following test program. It will crash with a ClassCastException. Exception
trace appended below.

Jasper says this is a bug in the PaintType converter.

import javafx.scene.*;
import javafx.scene.control.*;
import javafx.stage.Stage;

Stage {
    title: "Transparent Border"
    width: 600
    height: 600
    scene: Scene {
        stylesheets: [ "{__DIR__}transparent.css" ]
        content: [
            Separator {
                styleClass: "myseparator"
                layoutX: 50
                layoutY: 50
            }
            Separator {
                styleClass: "myseparator"
                layoutX: 50
                layoutY: 100
                vertical: true
            }
        ]
    }
}

===== transparent.css =====

/* this causes ClassCastException at runtime */
.myseparator *.line {
    -fx-border-style: dotted;
    -fx-border-color: derive(-fx-background,-20%) transparent transparent transparent;
    -fx-border-width: 1px;
    -fx-padding: 3;
    -fx-background-color: null;
}

===== exception trace =====

java.lang.ClassCastException: java.lang.Float
        at com.sun.stylesheet.css.Type$PaintType.getParsedValue(Type.fx:633)
        at com.sun.stylesheet.css.Type$PaintType.getParsedValue(Type.fx:633)
        at com.sun.stylesheet.css.Type$PaintType.convert(Type.fx:620)
        at com.sun.stylesheet.css.Type$PaintType.convert(Type.fx:620)
        at com.sun.javafx.scene.layout.region.StrokeBorder$BorderPaintType.convert(StrokeBorder.fx:215)
        at com.sun.javafx.scene.layout.region.StrokeBorder$BorderPaintType.convert(StrokeBorder.fx:215)
        at com.sun.stylesheet.StyleHelper.lookup(StyleHelper.fx:297)
        at com.sun.stylesheet.StyleHelper.lookup(StyleHelper.fx:297)
        at com.sun.stylesheet.StyleHelper.lookup(StyleHelper.fx:250)
        at com.sun.stylesheet.StyleHelper.lookup(StyleHelper.fx:250)
        at com.sun.stylesheet.StyleHelper.transitionToState(StyleHelper.fx:153)
        at com.sun.stylesheet.StyleHelper.transitionToState(StyleHelper.fx:153)
        at javafx.scene.Node.impl_processCSS(Node.fx:3071)
        at javafx.scene.Node.impl_processCSS(Node.fx:3071)
        at javafx.scene.Parent.impl_processCSS(Parent.fx:856)
        at javafx.scene.Parent.impl_processCSS(Parent.fx:856)
        at javafx.scene.Parent.impl_processCSS(Parent.fx:859)
        at javafx.scene.Parent.impl_processCSS(Parent.fx:859)
        at javafx.scene.Parent.impl_processCSS(Parent.fx:859)
        at javafx.scene.Parent.impl_processCSS(Parent.fx:859)
        at javafx.scene.Parent.impl_processCSS(Parent.fx:859)
        at javafx.scene.Parent.impl_processCSS(Parent.fx:859)
        at javafx.scene.Scene.doCSSPass(Scene.fx:285)
        at javafx.scene.Scene.doCSSPass(Scene.fx:285)
        at javafx.scene.Scene.userInit$(Scene.fx:598)
        at javafx.scene.Scene.userInit$(Scene.fx:598)
        at com.sun.javafx.runtime.FXBase.complete$(FXBase.java:288)
        at TransparentBorder.javafx$run$(TransparentBorder.fx:16)
        at TransparentBorder.javafx$run$(TransparentBorder.fx:16)