Update: Sorry, I thought you were asking how to trap Control-S in your webpage to save the file. Oh well, back to the white powder I found on the floor...

Then you will have to use client side javascript. It can be done, but not without some effort. You will need to look at trapping key events. For example, you can look at: http://tech.irt.org/articles/js195/

Also remember, you will want to test this on different browsers with different versions due to the differences/bugs in various Javascript implimentations. And of course, many people have javascript off and don't even know it.