action the relative url would sometimes become a fully qualified url depending on which method you used in which browser.
|action||Relative||Relative||Fully Qualified||Fully Qualified||Relative|
The end result of this little oddity is that you are best using fully qualified urls in form actions. However if you do need to use a relative url as an action you need to be careful to make sure that you end up with what you are expecting. For example I'd use getAttribute('action') and just fix the result coming from opera by checking for the presence of 'http'.
See the simple example