This is commonly the case for online shopping, where account creation is optional and may only occur after confirmation, but the order will be placed regardless.

But in that case, there is still an account (just without login ability), and you are probably even sending an email.

So in the email you can include a link to optional account (username/password) creation.

And if you aren't sending an email, then the user has clearly decided not to create an account.

The problem is if an account for that email already exists, see https://news.ycombinator.com/item?id=33720063.

See my comment https://news.ycombinator.com/item?id=33720405

You shouldn't be taking my email just to demand lots of information from me after I already gave my email to you.

If you demand lots of information that should be clear right away.

So now you're saying web UX should actually improve and give up the major disadvantage it has over paper forms: hiding the full flow from the (l)users.

/s, but only slightly.

You're right, of course. The answer is the one nobody wants to hear: you can tell users what you'll want from them, then ask to create an account, and then ask them to do the things you outlined before registration.

As for multi-page forms, they make sense if you target non-JavaScript use, but if your site is already an SPA, you might as well present the form in full, and conditionally disable parts that don't apply based on earlier inputs. This is the way to improve over the paper UX.

I can sympathize with your frustration about long web forms, but that seems to me like a separate issue with a separate solution.

People shouldn't be making long forms part of account creation unless absolutely necessary; in which case it should already be obvious to the user that it is.

If it's somehow necessary but not obvious, you can put up a friendly warning. Maybe something like, "Step 1/12" or "expected registration time 15min."