If a page contains a video tag, inside of which is an iframe pointing at a page to be displayed in browsers that do not support the tag, and the page pointed to contains a Java applet, Java is started and the applet runs. HTML inside the video tag should be ignored if the browser understands the video tag. Firefox and Opera both have this behaviour. The URL above is to a short demo of the bug - the applet inside the iframe just prints to the Java console and causes the machine to beep a few times.
Java runtime plugin support is gone and now HTML5 <video> element rules. Following Wayback Archive copy is available here: http://web.archive.org/web/20140808000433/https://wwwb.forbidden.co.uk/~nae/html5iframebug/ Nothing works in above test case. Considering it was about fallback from applet element, I think we can consider this now fixed since it is gone. Marking this as 'RESOLVED WONTFIX' (because it was never confirmed). Please reopen, if it still make sense to fix. CCing - Eric & Jer for their input.