[Xenon]

with css you have a lot of possibilities for textdesigns aso.
But from a userfriendly point of view i wouldn't use too much of them, as it's complicating.

"What means that, what means that aso" will be most common questions

As for Rainbowtext or like such: Yes doable by Javascript, but as Reeve said, i've no ide how ^^
but as said above, i wouldn't use too much effects, that's normally contraproductive