I've found the old thread about this. It's the one, that has many more suggestions:
There is an example of a site from german university-libraries, that uses exactly what I mean, but with Scripting:
E.g. if you activate the check-box "Deutschland" many checkboxes under it are activated too. The same with "Buchhandel", "Elektronische Volltexte", and so on.
This is what I mean. But it would be better, if no scripting is needed for this, but this should be possible only with HTML / CSS.