Anne van Kesteren

Cross-document messaging

HTML5 has a section which starts with:

Web browsers, for security and privacy reasons, prevent documents in different domains from affecting each other; that is, cross-site scripting is disallowed.

While this is an important security feature, it prevents pages from different domains from communicating even when those pages are not hostile. This section introduces a messaging system that allows documents to communicate with each other regardless of their source domain, in a way designed to not enable cross-site scripting attacks.

The cool thing here is that Opera has an implementation of that and that Arve has written about cross-document messaging in Opera. (With samples!)