HTMLEditorKitBugTest.java

import java.awt.BorderLayout;

import javax.swing.JEditorPane;
import javax.swing.JFrame;
import javax.swing.JScrollPane;
import javax.swing.JSplitPane;
import javax.swing.SwingUtilities;
import javax.swing.text.html.HTMLEditorKit;
import javax.swing.text.html.StyleSheet;

/**
 * Test application for HTMLEditorKit bug.
 */
public class HTMLEditorKitBugTest
{
    public static void main(String[] args)
    {
        SwingUtilities.invokeLater(new Runnable()
        {
            @Override
            public void run()
            {
                new HTMLEditorKitBugTest().run();
            }
        });
    }

    private void run()
    {
        JSplitPane split = new JSplitPane(JSplitPane.HORIZONTAL_SPLIT);
        split.setLeftComponent(new JScrollPane(createCustomisedEditorPane()));
        split.setRightComponent(new JScrollPane(createNormalEditorPane()));

        JFrame frame = new JFrame();
        frame.setLayout(new BorderLayout());
        frame.add(split);
        frame.setSize(600, 400);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);

        split.setDividerLocation(0.5);
    }

    private JEditorPane createCustomisedEditorPane()
    {
        JEditorPane editorPane = new JEditorPane();
        HTMLEditorKit editorKit = (HTMLEditorKit) editorPane.getEditorKitForContentType("text/html");
        StyleSheet styleSheet = editorKit.getStyleSheet();
        styleSheet.addRule("p { color: red }");
        editorPane.setContentType("text/html");
        editorPane.setText("<p>Should be red.</p>");
        return editorPane;
    }

    private JEditorPane createNormalEditorPane()
    {
        JEditorPane editorPane = new JEditorPane();
        HTMLEditorKit editorKit = (HTMLEditorKit) editorPane.getEditorKitForContentType("text/html");
        editorPane.setContentType("text/html");
        editorPane.setText("<p>Should be the default colour.</p>");
        return editorPane;
    }
}

Shows both the left and the right pane in red text. :/