304响应， cors问题: 没有 access-control-allow-origin 这个头信息时，以前次返回的200请求为准。
示例：可能已被删除
http://7af3zm.com1.z0.glb.clouddn.com/ajax_304_cors.html
附上源码：
html
<!doctype html><html><head> <meta charset="UTF-8"></head><body><script type="text/javascript">// var _ajax_34re = false; function getAjax () { var _ajax_34re = false; if (_ajax_34re) { return _ajax_34re; } try { _ajax_34re = new XMLHttpRequest(); } catch (trymicrosoft) { try { _ajax_34re = new ActiveXObject("Msxml2.XMLHTTP"); } catch (othermicrosoft) { try { _ajax_34re = new ActiveXObject("Microsoft.XMLHTTP"); } catch (failed) { _ajax_34re = false; } } } return _ajax_34re; } function load(url) { request = getAjax (); request.open("GET", url); request.onreadystatechange = updatePage; request.send(null); } function updatePage() { if (request.readyState == 4) if (request.status == 200) alert("Server is done!" + request.responseText); else if (request.status == 404) alert("Request URL does not exist"); else alert("Error: status code is " + request.status); }</script><pre class="brush:php;toolbar:false;"> access: 200、304响应都有 Access-Control-Allow-Origin:*; access-non: 200、304响应都 没 有 Access-Control-Allow-Origin; access-200: 200响应都有 Access-Control-Allow-Origin:*, 304响应 没 有 Access-Control-Allow-Origin;
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