function addInputSubmitEvent(form, input) {
    input.onkeydown = function(e) {
        e = e || window.event;
        if (e.keyCode == 13) {
            form.submit();
            return false;
        }
    };
}

window.onload = function() {
    var forms = document.getElementsByTagName('form');

    for (var i=0;i < forms.length;i++) {
        var inputs = forms[i].getElementsByTagName('input');

        for (var j=0;j < inputs.length;j++)
            addInputSubmitEvent(forms[i], inputs[j]);
    }
};