First things first: in my previous post I suggested that the use of frames would be better to be able to reload 'pieces' of pages. This is not completely correct of course: within a frameset multiple frames display pages, so in principle the page in a frame is reloaded although this page is of smaller width and / or height so that the frameset fits the screen.
And although the idea of frames is good (reloading parts of a website), it is not worked out great. You still have to reload a whole page. After a quick thought and some trying with Smarty it is way better to just use boxes as I already wanted to do. Let's say I was blinded by the 'reloading pieces of pages' idea 😉