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I've had Gogo's inflight wifi not let me sign up because my email account at gmail started with a number.

Same issue with our hi.fi. Facebook's JavaScript SDK wouldn't write cookies to it, instead rejecting it as too short and invalid. Had to open a ticket and the oversight was corrected, with lots of embarrassment and apologizing.

To be fair, an old RFC referenced by an RFC that is referenced by the current thing specifies that a segment of a domain name can't start with a number. Obviously, that isn't true in the real world.

So, TLDs are selling (technically) invalid domains?

And every major implementation supports them.

Well, I trudged through the RFCs in ~2010, so it's possible that it's been updated since then.