I haven't misunderstood. I simply disagree. If I am on a public web page of a particular resource I should be able to share the URL to anyone else. Neither my user agent nor the other party's user agent should matter. It is ludicrous (and in fact essentially impossible) to check how a URL behaves in every possible user agent before sharing that URL with someone. If I'm on the Wikipedia page for London, I ought to be able to share that URL with anyone using any user agent. If I cannot do that, that is the responsibility of the web site administrators.

> If I am on a public web page of a particular resource I should be able to share the URL to anyone else. Neither my user agent nor the other party's user agent should matter.

This is how mobile wikipedia pages behave. What part of that do you disagree with?

Share a mobile page's URL and the person following your link will see the same page you do, no matter what your user-agent is, no matter what their user-agent is, no matter what device you're using, no matter what device they're using.

This is the problem. The behavior you are looking for, of different people seeing different content according to their user-agent, is also provided, and it's provided by the standard page.

Again, the point is that the non-mobile URL redirects to the mobile URL. You're not "sharing a mobile page's URL," you're just sharing the URL for the page you're on. You didn't choose to be on a mobile-specific page. And again, your user agent does matter, because that's how you got to the mobile URL in the first place. Returning HTTP redirects is not supposed to mean "here's a new URL that's different from the one you requested, oh any by the way you can't share this new URL." An HTTP redirect is supposed to mean "the resource you requested is now at a new URL."

> Again, the point is that the non-mobile URL redirects to the mobile URL.

And I would argue that this is not only the point but the problem we are talking about. It makes the m. pages appear all over the Internet although they are less suitable for some devices (they are, I'd argue, even less usable because, e.g., the page history is missing when you are not logged in).