Sakhet,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<form>
Тег 1<input type="checkbox" name="tag1" value = "Тег1"><br>
Тег 2<input type="checkbox" name="tag2" value = "Тег2"><br>
Тег 3<input type="checkbox" name="tag3" value = "Тег3"><br>
<input type="submit" value="Отправить">
</form>
<script>
document.querySelector('form').onsubmit = function(e){
e.preventDefault();
var data = "cat=1";
var boxes = document.querySelectorAll('input[type="checkbox"]:checked');
if(boxes.length)
data += "&tag=" + [].map.call(boxes, function(el) {
return el.value
}).join("%2B");
alert(decodeURIComponent(data));
}
</script>
</body>
</html>