There are a number of email message headers that people may use to filter messages from W3C lists. The best header to use for this purpose is List-Id: (specified in RFC 2919), because this header is intended to remain constant throughout the lifetime of a list.
Using other headers to filter mail from our lists may not work as reliably, because the headers may change when we make changes to our email infrastructure or mailing list software.
Today we made a change to our list software to cause it to stop generating Sender: headers, because doing so was causing DKIM signatures to fail in some cases, resulting in messages bouncing and people being removed from our mailing lists.
If your email filters were based on the Sender header, please update them to use List-Id instead, as this will provide a more reliable and future-proof means of filtering.
We have a wiki page that provides info on how to filter messages based on List-Id in various email clients; additions welcome!
I can’t add stuff to the wiki, but FYI: the mail client in Opera automatically creates smart filters for any mailing list, based on the List-ID header.