Subject tagging for W3C lists

Occasionally we receive requests to modify the Subjects of messages as they pass through our mailing lists to add a list-specific prefix to the subject, e.g. for the www-html list:

Subject: [www-html] HTML Improvement/Suggestion

We do not do this for our lists, for a number of reasons:

All messages distributed by our mailing lists are sent with a List-Id header identifying the list, e.g. for www-html:

List-Id: <www-html.w3.org>

This is the standard way to identify a mailing list, per RFC 2919. Users who wish to filter mail from mailing lists into different mailboxes should do so using this header. Mail clients should use this header and the related headers specified by RFC 2369 to allow users to filter, join and leave mailing lists with minimal effort. If your mail client software does not provide these features, please file a bug report with your software vendor.
The subject tagging practice was started years ago as a workaround for mail clients that had no ability to filter mail into different mailboxes. Fortunately, most modern email software has built-in filtering capabilities, so it is rarely needed today.
The Subject of a message is one of the most important parts of the message; to use much of it up with metadata that is already better specified elsewhere in the message header is a waste of that precious space, and this impacts everyone on the mailing list, not just those with outdated mail clients.
Subject tagging is not a reliable way to identify mail from a mailing list versus personal email, because if someone sends a personal reply off the list, they will likely not remove the list tag from the subject and the mail will get filtered to the recipient's list mailbox even though it was not distributed by the list.
If certain users are unable to filter mail based on the List-Id header, our list software adds a number of other headers that may be used for filtering, e.g. Sender, X-Mailing-List, X-Loop, Resent-Sender, etc. However, these headers are less reliable than the List-Id header, which will always be present, so other headers should be used as a last resort.

Although we realize that there may still be some email clients that do not make it straightforward to filter based on any of these headers, we still do not provide a subject tagging facility for our lists because we would rather see effort invested in getting mail software fixed to do the right thing than provide workarounds that provide no incentive to do so, at the expense of users whose mail clients already do the right thing.