I think you have a little divitis
going on there. It seems to me that #nappula and its odd right margin don't help you at all. Why not do away with that and just position your anchor where you want it to be?
A big part of your problem is not having the same CSS styling your back button and it's container. On some pages the CSS comes from Bulkse.css and other pages from kategoriat.css.
You style #content differently from those two CSS files as well, there may be plenty of other conflicts but I only mention those two.
Yes, you have invalid code and you should work on fixing that. The way you are positioning that back button still won't work right, even after you validate your code. Valid code doesn't necessarily mean working code.