> Subscribers to my World Wide Words newsletter now often ask whether 
> it is possible to receive it in HTML format, rather than plain text. 
> At the moment, it only goes out in that format via RSS, but it would 
> be trivial to create a related version for the LISTSERV (both the RSS 
> and Web page versions are output from a database). 
> 
> There would seem to be various possibilities, on the basis that a lot 
> of people would continue to want the text version:
> 
> a) create a second list for HTML subscribers
> 
> b) stick with one list but send two copies of the newsletter, using 
> Topics to distinguish them
> 
> c) send one message containing both formats (though I'm not at all 
> sure how to do this using my Pegasus mailer, nor whether the LISTSERV 
> would accept it; it's version 14.4).
> 
> d) something else entirely ...
> 
> Has anyone experience with sending two versions of newsletters, and 
> which would be the recommended way forward? (I know that many members 
> of this list would urge me to avoid HTML, and it's my preference, 
> too. But I'd like at least to explore the options, if there are any.)

You covered all your options above, each has it's own pros and cons.

"a" would be less confusing to the subscribers. You of course need two lists 
now and need to send two newsletters.

"b" Topics can be confusing for both users and owners. So, maintaining 
subscriptions and user options will probably take more from you. You still need 
to send two newsletters and more distribution mistakes are possible because of 
the Subject line requirements of Topics.

"c"  Only requires you send one message as a multipart/alternative and no 
changes to LISTSERV. (unless you have a message size restriction or Language= 
NOHTML is set)

From here my question would be why on earth do these people want HTML. I have 
to assume because this is "News" your messages only needs to be text, whether 
it's plain or fancy. So why someone would want to see green Comic Sans text on 
a purple background is beyond me. 

As far as Pegasus and "c", create an identity using the allowed posting 
address. Use "MIME features - On" and "Rich (formatted) text - On". Under 
"formatting and use of styling in message" uncheck all 3.  Under "Sending 
mail", check "Generate multipart/alternative versions of richtext messages" 
If you opt for "a" or "b",  un-check this option.

Now you can compose your fancy version in Pegasus, or you can simply cut from 
another application and paste into the message. When Pegasus sends the message 
it will create the plain text portion from the text in the fancy version.

I much prefer "c" over the others. The only "gotcha" would be if some were 
using a non-MIME mail client. In that case you're stuck with "a" or "b" unless 
you want to manipulate the Pegasus mail file on disk.