See test at http://w3c-test.org/html/semantics/forms/the-button-element/button-click-submits.html and spec at https://html.spec.whatwg.org/#form-submission-algorithm
After further discussion, I think I filed this bug prematurely. We are still in discussion as to whether form should have a connected check in https://github.com/whatwg/html/issues/2615, because we want to consistify it with a/area, and Chrome is unsure yet if they're willing to do that, pending use-counter data. So it's probably best to hold off on changes; there's a chance (although hopefully small) that it will end up being wasted effort. I will ping this bug again when it is “safe” to change (i.e. definitely own’t be wasted effort). Apologies about this.
We've figured out the spec story here. There are two connected checks, one in step 1 and one in step 6.3. Additional tests are available at http://w3c-test.org/html/semantics/forms/form-submission-0/submission-checks.window.html; Safari TP 43 at least doesn't seem to have either check implemented.