MacOS Sierra uses the San Francisco system font, and occasionally this font is used in bold. In JavaFX, however, creating a bold label using the default system font does not work on mac, as the following simple test case shows. I am guessing this has something to do with how a font weight of "bold" translates to the actual font selected. I do think this is a serious bug from a user experience stand point, as JavaFX cannot visually mimic Apple's design guidelines without jumping through hoops.

public class BoldTest extends Application {

@Override
public void init() throws Exception {
}

@Override
public void start(Stage stage) throws Exception {
Label normal = new Label("Normal");
Label bold = new Label("Bold");
bold.setStyle("-fx-font-weight: bold;");
HBox box = new HBox();
box.getChildren().addAll(normal, bold);
Scene scene = new Scene(box);
stage.setScene(scene);
stage.setTitle("Bold test");

stage.setWidth(600);
stage.setHeight(600);
stage.show();
}

public static void main(String... arguments) {
launch(BoldTest.class, arguments);
}
}