Similarly, as you say, the host software provider has no incentive to require
this
functionality: for the most part, it simply makes life
difficult for the end user. At present, Google is actually pretty good
about this; the result is that it's hard to use Google devices on
networks with broken captive portals, which are of course endemic.
Google seems not to have paid a price for this thus far, but I don't see other
vendors doing what Google is doing.
It may well be one of these cases where we should disregard Jon Postel's advice
and NOT be tolerant with what we receive. Not being tolerant is a known way to
force change. Suppose that a critical fraction of devices refuses to use broken
portals, fail, call the help desk, etc. That will certainly create pressure on
portal vendors to fix the broken pages. And having the advice in the RFC will
provide cover to device vendors.
-- Christian Huitema