Ok, Thank you all for your answers, but if someone gets in this case here's how I solved it:
The #left 's height has a relation of the 50% of its width. So, because the width loads with the whole document, I put this js (With jQuery...):
$("#right").css("height",$("#left").width()/2);
And... that's it :)
Thanks to T J (Who made me realize it) and for all those who answered :D