Tests at http://w3c-test.org/html/semantics/forms/the-form-element/form-action-reflection.html and http://w3c-test.org/html/semantics/forms/the-form-element/form-action-reflection-with-base-url.html Spec at https://html.spec.whatwg.org/multipage/forms.html#dom-fs-action Per the separate tests at http://w3c-test.org/html/semantics/forms/the-form-element/form-action-submission.html and http://w3c-test.org/html/semantics/forms/the-form-element/form-action-submission-with-base-url.html, Safari uses the spec's algorithm for determining the actual URL to which the form submits. However, it does not return the correct URL for form.action. No browser seems to follow the spec here, but they violate it in different ways, so this is probably safe to change.
Theses tests now pass, at least partially due to https://bugs.webkit.org/show_bug.cgi?id=213205 fix.