SeBa,

>Who can me direct to the right way to make a HTML page expiring after leaving the page.
>
>I.e. if the user leaves the page to the next page and than pushs the browsers Back button he should get a system message like "This page is expired and is invalid..."!

I'm not sure if this is exactely what You need but it may get You on the way.
You can add meta-tags in the header that tell the Server when the page expires
that can be a date/time that already has passed by or 0 (and future vaulues of course)

You can find more here:

http://www.netzwelt.com/selfhtml/html/kopfdaten/meta.htm#laden

However this will (if it works) not necessarily display a warining. when
coming back to the page. However it should reload the page from the server
witch then could via session handling return a different page or a warning.

You might need to play with it a little.

Gruß aus Berlin nach Berlin (3°C gray sky)