Load WFS and use stylemap with external graphic

HTML

<script src="http://cdnjs.cloudflare.com/ajax/libs/openlayers/2.11/OpenLayers.js"></script>
<p id="demo"></p>

CSS

#map {
    width: 400px;
    height: 400px;
    border: 1px solid #dddddd;
}
.Panel2 {
    width: 18px;
    height: 54px;
    cursor: pointer;
    right: 10px;
    top: 74px;
}
.firstItemInactive, .secondItemInactive {
    background: #30d5c8;
    border: 2px solid #075f67;
    width: 18px;
    height: 18px;
}
.firstItemActive, .secondItemActive {
    background: #ffffdd;
    border: 2px solid #dddddd;
    width: 18px;
    height: 18px;
}

JavaScript

var xhttp = new XMLHttpRequest();
xhttp.onreadystatechange = function() {
    if (this.readyState == 4 && this.status == 200) {
        myFunction(this);
    }
};
xhttp.open("GET","http://geodata.nationaalgeoregister.nl/ahn2/wfs?request=getCapabilities", false);
xhttp.send();

function myFunction(xml) {
    var xmlDoc = xml.responseXML;
    document.getElementById("demo").innerHTML =
    xmlDoc.getElementsByTagName("title")[0].childNodes[0].nodeValue;
}