The document provides an overview of secure web messaging in HTML5. It discusses how traditional methods of communication like JavaScript, AJAX, and frames had limitations due to the same-origin policy. The HTML5 postMessage API allows for secure cross-origin communication between frames by abstracting multiple principals. While more secure than previous techniques, the postMessage API still requires careful configuration of target origins, validation of received data, and mitigation of framing attacks to prevent security issues like cross-site scripting.