FF或将支持返回一个HTML文档对象

function getDocument (aURL, aCallback) {
var xhr = new XMLHttpRequest;
xhr.open("GET", aURL, true);
xhr.responseType = "document";
xhr.onreadystatechange = function() {
if (xhr.readyState === xhr.DONE /* 4 */ &&
xhr.status === 200) {
aCallback(xhr.response);
}
};
xhr.send(null);
}

getDocument("http://www.example.com", function (doc) {
console.log(doc.querySelector("h1").innerHTML);
});