If you look at the results pane for a fiddle you'll notice that it's actually an IFRAME pointing to a different domain which means that built in security will kick in which generally prevents access to the parent window.
This fiddle for example : http://jsfiddle.net/jomanlk/y9zCK/
Is actually served by : http://fiddle.jshell.net/jomanlk/y9zCK/show/
