Explanation
We are using a javascript conditional statement for some alternative modes of presentation.
IF
In the IF part of the conditional, we specify an object document.documentElement, which is the top object of the DOM tree structure.
Browsers that recognize it should enter this part of the script and should produce the DOM effects.
Browsers such as NetNav4.x and Opera6.x, etc., will not recognize it.
They should hop over the IF part of the script.
ELSE
Earlier browsers (non-DOM) should enter into the ELSE part of the script.
Here, we could write the same material for an alert message as we would write for our DOM handled element.
The alert object is an earlier javascript object.
So, visitors with earlier graphical-browsers should obtain the same content in this mode of presentation, as an alert message.
Actually, the alert object is useful itself. We might want to use it sometimes for all of our javascript-enabled visitors, for everyone alike.