title: Sending a message to all open windows/tabs using JavaScript
body: <p>I hear HTML5 has <code>window.postMessage()</code>, but it seems to require having a handle on the window (or tab, throughout this question) you're posting the message to.  What if I want to broadcast to all open windows?  Is this possible?</p>
  
  <p>(What I'm trying to do is warn other windows without any server round-trips when a user does something in one window that affects the others, so that they can update their content.  However, while some windows may be opened from existing ones--allowing me to intercept and store references to them--some fresh windows may be opened manually by the user and then a bookmark selected or URL typed in.  In this case there doesn't seem to be a way to intercept and store references.)</p>