Reference: HTML 4.01 spec, section 17.5 The BUTTON element states that the default type of button is submit.
Firefox 1.x, Firefox 2.x, Firefox 3.x, Opera 9.25, Opera 9.50, Safari 3.0.4, Konqueror 3.5.8, Seamonkey 1.x, Seamonkey 2.x, NS 7.2, K-meleon 1.x, Galeon 2, Epiphany 2 all pass this test.