Comment by jefftk

> Is there any downside to being on this list?

If example.com were on list then a cookie set on a.example.com couldn't be read on b.example.com. In this case that would probably be a good thing, since the subdomains represent independent sites, but if a site were erroneously added that could be a problem (mail.yahoo.com and groups.yahoo.com should share login cookies, for example).

The list was originally created to handle cookies, but more recently it's been used for other notions of "site", like cache sharding.