The account creation form on mobile is exactly the same code as desktop.
To be clear.. I'm not seeing a "Go" button.. do you mean the "Create account" button? (maybe Go in another language version?)

But, if I'm not mistaken this is also an issue in https://en.wikipedia.org/wiki/Special:CreateAccount (just more noticeable on a mobile browser) (If so should we remove MobileFrontend and tag Mobile)?

Sorry, I should have been clearer. It's not a button that appears in the website's interface, but a key in the mobile keyboard itself. I edited the task description.

It's at the bottom right corner of this screenshot:

It can have other names if the phone's UI language is not English; I set mine to English especially to make the screenshot :)

When you hit go it is the same as hitting "create account" so yes the issue is also a problem with desktop - just more painfully experienced on mobile.
A really trivial move would be to add required attributes to all required fields. I'm surprised this hasn't been done already (most mobile browsers will then do validation for you)
@Volker_E what do you think?

I don't see any reason why we wouldn't add required attributes where they apply.

Jdlrobson some clarification requested since you added the Easy tag. All that needs to be done is that

if (strlen($pass||username||confirmuserpass) == 0 {
  echo "One of the requires fields are empty";
  break;
}

Any comments?

@DatGuy even more simple... a required attribute would be added to the required fields where they are generated.
e.g.

<input required name="captcha" />

Note that we already have server side validation - the goal is to prevent the user from submitting a form that is not complete.

@Jdlrobson One thing we need to assure is, to hide asterisks on those specific fields when we're invoking required.

Wait.. I'm seeing required fields and when I try to sign up in mobile it now prompts me to fill in required fields. Did something change / a patch get merged?

Maybe T155640 can shed some light. Is this fixed?

I think @matmarex may have already fixed this with Ia7ffa76965a7c14af9b6d2db007b6255498398d9

In the account creation form the user must write the new username, the password (twice), and the CAPTCHA (the email address is not required). All of these are required, and if the form is submitted without any of them, it is reloaded and the fields that were already filled, must be filled again.

The password (and confirmation) are cleared from output intentionally: "don't put passwords into the HTML body, they could get cached or otherwise leaked" (quoting the comment by the code that controls this behavior, HTMLTextField::isPersistent()). I think this is a commonly accepted good practice (although I've seen some sites not do it, so perhaps not).

The captcha text is cleared because you'll need to enter a new captcha if the submission fails. This is suboptimal of course, perhaps we should remember that the user filed in a captcha correctly once and not ask them again for some time (but there has to be a limit, otherwise the captcha's protection against bots is lost). But that would be a separate task.

All other fields (username and email) are preserved on a failed submission, as far as I can see.