I can see a reason for opening a text file in an iframe. And that is,
if you want to write a "News Bulletin". You could name each one with a
date, and only the one with today's date is loaded. This way, the
website owner can keep track of all the different bulletins for each
day, but the user only sees the latest.
I have a similar setup on my website
http://tandcl.homemail.com.au ,
although I haven't yet pursued the idea of dated text files - I only
thought of it as I was wriitng the reply :-)
Re the original question. I am not sure CSS is the way to go. I use JS.
I would be interested to see whether the OP or anyone else has any
further thoughts on using CSS