You might have a look at UBB-Code, it is explained on http://ubbforums.infopop.com/cgi-bin/ultimatebb.cgi?ubb=ubb_code_page and uses a small set of tags to format input.

I'm not sure if this can be considered a standard markup for what you need, but it looks pretty simple and a lot of sites use it.

alex pleiner <alex@zeitform.de>
zeitform Internet Dienste