Well you might be able to do this with server-side scripting but I am not sure. Even if you knew of a site that did this, you would not be able to check it out because it would shut-down you're computer. If it is done with server-side scripting, you won't be able to find out how they did it anyway. If you do find out how to do it, don't do this to people.
AFAIK it can't be done. Though this post has made me a little curious. If it were to be done I'd have to assume that the particular user had a special scripting interpreter installed. For example, PerlScript. Not many have it installed on their system, and (I guess because it is less common ) there are very little (if any) restrictions to what it can access on the client machine. It can even run commands on that system, such as format, or anything else.
Not that i know much (newbie)but, there is a computer program that you can run which uses a code to shut down your computer. I guess the same code as (STATRTUP+SHUTDOWN). Whoever made the site would probably know the code and put it on his site- or if you had to click on a link before it shut you down, uploaded this file so when you click on the link it runs the program.
There are at least couple of possible ways that I know of which will allow you to get this done.
This was th eonly reason why java applets were not very popular in the masses because they could give a script or java application running in the client browser (Applet) power to perform tasks on the client machine,hence the advent of term signed applet,that specifically have a customizes an applets rights on a ny given client.
2.A customised ActiveX Control will let you do the same on any given client browser(IE only) to perform tasks on that specific machine.