OK I make it more complicated as it realy is. All I need to do is hide (display:none) div which I don't want to validate. I did know that it don't validate hidden fields but I was thinking that hidden must be directly input tag (or select tag). But it is enough to hide some adjecent tag. For example:
<div id="form-step-2" class="hidden">...some fieldset, input etc...</div>
Now it will not validate form-step-2
div.
Anyway I still don't know why my previous approach isn't working...