UPGRADE: Feature detection?

While writing the example flow at
https://w3c.github.io/webappsec/specs/upgrade/#examples, I stumbled over
the problem of knowing when to redirect a user from an HTTP page to an
HTTPS one. If you require the upgrade mechanism we're defining in order to
give a user a reasonable experience, then you need to know whether or not
she's capable of performing the upgrade before redirection.

I think we should explicitly support this sort of feature detection, rather
than relying on user agent sniffing*. Perhaps something like the following
HTTP request header could be sent along with every navigational request
(e.g. top-level navigations, new windows, and iframes):

    Accept-Upgrade: https

Servers could inspect the headers of the request, and decide based upon the
presence of that header whether or not they were dealing with a client that
could transparently upgrade requests. If so, redirect to HTTPS if you're
not already there, if not, redirect to HTTP.

WDYT?

*Eric Mill and Jacob Hoffman-Andrews noted thre issues with UA detection in
https://lists.w3.org/Archives/Public/public-webappsec/2015Feb/0052.html and
https://lists.w3.org/Archives/Public/public-webappsec/2015Feb/0049.html,
but I didn't recognize the impact at the time. Sorry for the delay!

--
Mike West <mkwst@google.com>, @mikewest

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