Per-mailing-list subject-line tags are an abomination. Use of proper
mailing list management software (which provides a consistent "To:"
line, doesn't munge the headers, etc.) and use of any reasonable
filtering software on the part of the recipients alleviates the need
for them, thus providing a vastly more elegant and scalable solution.
(Consider that there are already at least three mailing lists about
HTML. Which one should get to use [HTML] as the tag? What if they
all want to?)
Either people get so much mail they need to filter, or they don't. People
who need to filter, should, and if they insist on not doing so, then
that's *their* problem, not the mailing list managers'.