To reproduce run following code

import javafx.application.Application;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class PathCss extends Application {

    public static void main(String[] args) {
        launch(args);
    }

    private Parent getContent() {
        VBox list = new VBox(10);

        Button but=new Button();
        but.setStyle("-fx-shape: M 100 100 L 300 100 L 200 300 Z;-fx-background-color: green;");
        list.getChildren().add(but);
        return list;
    }

    public void start(Stage stage) {
        stage.setX(100);
        stage.setY(100);
        stage.setWidth(300);
        stage.setHeight(300);
        Scene scene = new Scene(getContent());
        stage.setScene(scene);
        stage.setVisible(true);
    }
}


com.sun.javafx.svgpath.parser.ParseException: character.unexpected
        at com.sun.javafx.svgpath.parser.AbstractParser.reportError(AbstractParser.java:172)
        at com.sun.javafx.svgpath.parser.AbstractParser.reportUnexpectedCharacterError(AbstractParser.java:198)
        at com.sun.javafx.svgpath.parser.NumberParser.parseFloat(NumberParser.java:55)
        at com.sun.javafx.svgpath.parser.PathParser.parseM(PathParser.java:145)
        at com.sun.javafx.svgpath.parser.PathParser.doParse(PathParser.java:85)
        at com.sun.javafx.svgpath.parser.AbstractParser.parse(AbstractParser.java:121)
        at com.sun.javafx.svgpath.parser.Path2DProducer.createPath(Path2DProducer.java:35)
        at com.sun.javafx.tk.quantum.QuantumToolkit.createSVGPath2D(QuantumToolkit.java:1036)
        at javafx.scene.shape.SVGPath.impl_configShape(SVGPath.java:124)
        at javafx.scene.shape.SVGPath.impl_configShape(SVGPath.java:33)
        at javafx.scene.shape.Shape.impl_computeGeomBounds(Shape.java:521)
        at javafx.scene.Node.updateGeomBounds(Node.java:2449)
        at javafx.scene.Node.getGeomBounds(Node.java:2411)
        at javafx.scene.Node.getLocalBounds(Node.java:2393)
        at javafx.scene.Node.updateTxBounds(Node.java:2462)
        at javafx.scene.Node.getTransformedBounds(Node.java:2323)
        at javafx.scene.Node.impl_updatePG(Node.java:393)
        at javafx.scene.shape.Shape.impl_updatePG(Shape.java:603)
        at javafx.scene.shape.SVGPath.impl_updatePG(SVGPath.java:134)
        at javafx.scene.Node.impl_syncPGNode(Node.java:372)
        at javafx.scene.layout.Region.impl_updatePG(Region.java:1381)
        at javafx.scene.Node.impl_syncPGNode(Node.java:372)
        at javafx.scene.Scene$ScenePulseListener.syncAll(Scene.java:1431)
        at javafx.scene.Scene$ScenePulseListener.syncAll(Scene.java:1440)
        at javafx.scene.Scene$ScenePulseListener.syncAll(Scene.java:1440)
        at javafx.scene.Scene$ScenePulseListener.synchronizePGScene(Scene.java:1393)
        at javafx.scene.Scene$ScenePulseListener.pulse(Scene.java:1517)
        at com.sun.javafx.tk.Toolkit.firePulse(Toolkit.java:268)
        at com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:350)
        at com.sun.javafx.tk.quantum.QuantumToolkit$9.run(QuantumToolkit.java:286)
        at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
        at com.sun.glass.ui.win.WinApplication.access$100(WinApplication.java:29)
        at com.sun.glass.ui.win.WinApplication$1$1.run(WinApplication.java:49)