Rorbi,
строка 16 неверно указан класс
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<div class="kabinet_selectid">
<select class="selectid" id="nazn" required="required" name="nazn">
<option disabled value="">Выберите специализацию:</option>
<option class="sel_dok" value="1" id="one1">Первый</option>
<option class="sel_dok" value="2" id="two1">Второй</option>
<option class="sel_dok" value="3" id="three1">Третий</option>
</select>
</div>
<div class="clear"></div>
<div style="display:none" class="one">
<p>Первый блок</p>
</div>
<div style="display:none" class="too">
<p>Второй блок</p>
</div>
<div style="display:none" class="three">
<p>Третий блок</p>
</div>
<div class="block11">
<div style="display:none" class="one">
<p>Первый блок</p>
</div>
<div style="display:none" class="too">
<p>Второй блок</p>
</div>
<div style="display:none" class="three">
<p>Третий блок</p>
</div>
</div>
<script type="text/javascript">
window.onload = function () {
var opt = (window.location.search).replace("?", ""),
select = document.getElementById("nazn"),
options = select.options;
for (var i = 0; i < options.length; i++) {
if (options[i].id == opt) {break;}
}
i = 2 //убрать
select.selectedIndex = i;
function go() {
var b = {
1: ".one",
2: ".too",
3: ".three"
}, c = this.value,
a;
for (a in b)
{var elems = document.querySelectorAll(b[a]);
Array.prototype.forEach.call(elems, function(el)
{
el.style.display = c == a? "block": "none"
}
);
}
};
select.onchange = go;
go.call(select)
}
</script>
</body>
</html>