(Warning: what follows may be obvious or trivial to many.)
One of the cool things about AJAX is switching parts on and off: you
can make an element visible simply by
myElement.style.display = "block";
or hide it with
myElement.style.display = "none";
know a lot about the document. The example above doesn’t look too bad,
but what if you have something like a pulldown menu that appears when
you click a button?
Let’s say that originally, the button is gray and has a “+” icon next
to the text. When you click on it, the menu becomes visible, but the
button also changes to red, and the “+” icon changes to “-“, to show
that the menu is active.
Now you have all sorts of CSS resources that you have to keep track
of. It would be nice to put them in the .css file, with the
rest of the style stuff.