org.openide.awt.SwingBrowserImpl.run() will do some UI operation on non AWT thread. In an application with EDT violation monitoring (e.g JDEV debug version), it would trigger exception:
EDT Violations:May 01, 2015 9:08:45 AM oracle.javatools.internal.ui.SwingThreadViolationDetector$Violation report
  INFO: Call to Swing component not from the EDT Thread - in violation of Swing's single threaded policy.
  javax.swing.JLabel[,0,586,400x14,alignmentX=0.0,alignmentY=0.0,border=,flags=8388608,maximumSize=,minimumSize=,preferredSize=,defaultIcon=,disabledIcon=,horizontalAlignment=LEADING,horizontalTextPosition=TRAILING,iconTextGap=4,labelFor=org.openide.awt.HtmlBrowser[,0,0,400x600,invalid,layout=java.awt.BorderLayout,alignmentX=0.0,alignmentY=0.0,border=,flags=9,maximumSize=,minimumSize=,preferredSize=],text=Document: Done,verticalAlignment=CENTER,verticalTextPosition=CENTER]
  	at javax.swing.JComponent.repaint(JComponent.java:4802)
  	at java.awt.Component.repaint(Component.java:3303)
  	at javax.swing.JLabel.setText(JLabel.java:345)
  	at org.openide.awt.HtmlBrowser$BrowserListener.propertyChange(HtmlBrowser.java:703)
  	at java.beans.PropertyChangeSupport.fire(PropertyChangeSupport.java:335)
  	at java.beans.PropertyChangeSupport.firePropertyChange(PropertyChangeSupport.java:327)
  	at java.beans.PropertyChangeSupport.firePropertyChange(PropertyChangeSupport.java:263)
  	at org.openide.awt.SwingBrowserImpl.setStatusText(SwingBrowserImpl.java:372)
  	at org.openide.awt.SwingBrowserImpl.run(SwingBrowserImpl.java:348)
  	at org.openide.util.RequestProcessor$Task.run(RequestProcessor.java:1425)
  	at org.netbeans.modules.openide.util.GlobalLookup.execute(GlobalLookup.java:68)
  	at org.openide.util.lookup.Lookups.executeWith(Lookups.java:303)
  	at org.openide.util.RequestProcessor$Processor.run(RequestProcessor.java:2038)

 Click a button whose actionListener would perform the following code:
  HtmlBrowser browser = new HtmlBrowser();
          browser.setURL(url);
  
          JFrame frame = new JFrame();
          frame.setDefaultCloseOperation(WindowConstants.DISPOSE_ON_CLOSE);
          frame.getContentPane().add(browser);
          frame.pack();
          frame.setVisible(true);
  
  url points to a html file inside jar: url = MyClass.class.getClassLoader().getResource("com/oracle/.../MyPage.html");
  The html did get displayed but it also throw the EDT violation.