HTML5 Field validation is not invoked on external form submit

[ackvf]

Recommended practices here https://github.com/final-form/react-final-form/blob/master/docs/faq.md#how-can-i-trigger-a-submit-from-outside-my-form
don't trigger HTML5 validation, luckily another method can be used to work around the issue.

I wonder if this is design limitation or a bug.

https://codesandbox.io/s/7kk9w2ryzq