I'm not sure what you mean -- how can the browser possibly break the sites security?

I understand that English is not everyone's first language, but I honestly had a hard time parsing the linked post.

Sandbox iframes allow disabling javascript in a frame, which disables framebusting protection [1] used by sites like vk.com. The better way to framebust is to add the header 'x-frame-options: deny', which isn't broken by html5 sandboxes.

[1] http://en.wikipedia.org/wiki/Framekiller

Obviously he meant turning of JS made clickjacking feasible again for many websites. Why u pretend to not understand that? Are u kind of html5 moralist?