Thanks for the tip - switching to Yahoo Mail Classic did fix the problem. Sam B. suggested that the problem might be with ARSCList, and that I should send the message SET ARSCLIST NO HTML to [log in to unmask]
I suspect that you are both right. Apparently ARSCList and only ARSCList reacts badly to the new Yahoo mail interface (I get three other listservs here and they were not affected).
So if anyone else is having a problem with messages cluttered with <BR>   <a href= and the like, you might try one of these solutions.
--- On Fri, 12/3/10, Kevin Schlottmann <[log in to unmask]> wrote:
From: Kevin Schlottmann <[log in to unmask]>
Subject: Re: Why are ARSCList messages garbled?
To: [log in to unmask]
Date: Friday, December 3, 2010, 1:35 PM
Dear Mr. Brooks,
I've been having the same display issue with a few different listservs
lately. I haven't tried to thoroughly troubleshoot it yet, but since
it happens in both Firefox and Chrome, I believe it is a problem in
Yahoo's new mail interface. (I'll take a guess that you are using the
(generally) superior new mail interface). I was able to fix it by
switching to the old yahoo mail; you could try going to options (in
the upper right corner and then choose "Switch to mail classic." And
then send a message to Yahoo, letting them know you are having this
issue. Good luck.