As I was exercising the forms designer, ran across two fundamental bugs in its output, one of which is bad enough, but the other results in invalid HTML markup being emitted, which means there's no guarantee it will actually work in any particular current or future browser. Right now it works simply because of Postel's Law. Spec changes could break it, and there's no guarantee future User Agent developers will support it, because it's never been supported in a spec. (Even the HTML4 spec prohibited it; the fact it works the way it does is merely an accident based on ease of implementation.)
Invalid HTML bug:
The forms designer locates the form inside a span element, and also inside a noscript element. Both locations are forbidden to a form according to the HTML 5 specification. The form element is type 'Flow Content', the span element is to contain only 'Phrasing Content' (and it's also invalid inside the noscript element, since the noscript element is located within a span, and the content type of noscript is 'Transparent,' meaning it takes on the content type of its parent).
The other bug is that it refuses to allow a submit button to be placed inside a fieldset, something which is both explicitly allowed by the HTML 5 spec (a submit button is merely a subclass of an input field or a button element, depending upon markup, and both are allowed within a fieldset) and extremely useful for setting a form apart from normal page content, both visually and for assistive technology, due to the legend element. This second bug can be compensated for visually with some creative CSS, but it loses its association with the form and the fields within it. Might be able to workaround it for assistive technologies with ARIA roles, but hey, that's right, you're also not allowed to assign ARIA roles to form elements in the forms designer, are you?