I saw this and thought it looked quite nice so I ran some tests on it.
It may seem like a clean approach, but in terms of performance it is lagging by 50% compared by the time it took to load a page with jQuery load function or using the vanilla javascript approach of XMLHttpRequest which were roughly similar to each other. 
I imagine this is because under the hood it gets the page in the exact same fashion but it also has to deal with constructing a whole new HTMLElement object as well. 
In summary I suggest using jQuery. The syntax is about as easy to use as it can be and it has a nicely structured call back for you to use. It is also relatively fast. The vanilla approach may be faster by an unnoticeable few milliseconds, but the syntax is confusing. I would only use this in an environment where I didn't have access to jQuery.
Here is the code I used to test - it is fairly rudimentary but the times came back very consistent across multiple tries so I would say precise to around +- 5ms in each case. Tests were run in Chrome from my own home server:
<!DOCTYPE html>
<html>
<head>
    <script src="https://code.jquery.com/jquery-2.1.4.min.js"></script>
</head>
<body>
    <div id="content"></div>
    <script>
        /**
        * Test harness to find out the best method for dynamically loading a
        * html page into your app.
        */
        var test_times  = {};
        var test_page   = 'testpage.htm';
        var content_div = document.getElementById('content');
        // TEST 1 = use jQuery to load in testpage.htm and time it.
        /*
        function test_()
        {
            var start = new Date().getTime();
            $(content_div).load(test_page, function() {
                alert(new Date().getTime() - start);
            });
        }
        // 1044
        */
        // TEST 2 = use <object> to load in testpage.htm and time it.
        /*
        function test_()
        {
            start = new Date().getTime();
            content_div.innerHTML = '<object type="text/html" data="' + test_page +
            '" onload="alert(new Date().getTime() - start)"></object>'
        }
        //1579
        */
        // TEST 3 = use httpObject to load in testpage.htm and time it.
       function test_()
       {
           var xmlHttp = new XMLHttpRequest();
           xmlHttp.onreadystatechange = function() {
                if (xmlHttp.readyState == 4 && xmlHttp.status == 200)
                {
                   content_div.innerHTML = xmlHttp.responseText;
                   alert(new Date().getTime() - start);
                }
            };
            start = new Date().getTime();
            xmlHttp.open("GET", test_page, true); // true for asynchronous
            xmlHttp.send(null);
            // 1039
        }
        // Main - run tests
        test_();
    </script>
</body>
</html>