[danik.js]

var chk1 = document.querySelector('input[name="chk1"]');
var chk2 = document.querySelector('input[name="chk2"]');

var next = document.querySelector('#button-next');

next.addEventListener('click', function(e){

if (chk1.checked && chk2.checked) {
    alert ('wtf?');
    return false;
}

if (!chk1.checked && !chk1.checked) {
    alert ('wtf2?');
    return false;
}

scr1.style.display='none';
if (chk1.checked) {
    scr2.style.display='block';
} else {
    scr3.style.display='block';
}

});