They are either using an extension to the browser (likely IE) which is a
bad idea, or they are using javascript and checking the length of the
field on each key press which works reasonably well.
Check on some of the javascript cookbook sites and you will likely find
this type of thing, if you
How can I create a multilane textfield and limit the max number of characters?
(maxlength="100" doesn't work)
I've seen some pages where they have a "live" number of characters that decreases as
you type your message and stops (also beeps) when you reach the max number of
characters allowed.
