The standard JSF implementation doesn't provide facilities for this out the box. Besides, this is more a client side issue than a server side issue, so you'll really need to grab a client side language for this such as JavaScript. Since writing crossbrowser compatible JavaScript code for this particular functional requirement isn't exactly trivial, you'd like to use a JavaScript library for this which takes this into account, like jQuery.
Here's a complete kickoff example of how you could achieve this with help of jQuery.
Just paste this in your
