Is this possible? Basically, I want to be able to set up a tag that says "within this tag, a-z go into italics, but nothing else does." Or similarly for a-z and A-Z.
CSS is meant to style the structure of the page not the content of the page directly.
This being said, there are a couple of ways around it (obviously):
<i>,<span>,<em>,<dfn>,<var>,<cite>
There is no way to pick specific characters using CSS.