jQuery getJSON Remake

var $ = function (query) {
    var ritem = document.querySelector(query);
    ritem.css = function (key, value) {
        document.querySelector(query).style[key] = value;
    };
    ritem.click = function(handler){
        document.querySelector(query).onclick = handler;
    };
    return ritem;
};
$.getJSON = function (json) {
    var xmlhttp;

    if (window.XMLHttpRequest) {
        // code for IE7+, Firefox, Chrome, Opera, Safari
        xmlhttp = new XMLHttpRequest();
    } else {
        // code for IE6, IE5
        xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
    }

    xmlhttp.onreadystatechange = function() {
        if (xmlhttp.readyState == 4 ) {
           if(xmlhttp.status == 200){
               if (xmlhttp.getResponseHeader("content-type").indexOf('json') > -1){
                   json.success(JSON.parse(xmlhttp.responseText));
               }
           }
        }
    }

    xmlhttp.open("GET", json.url, true);
    xmlhttp.send();
};