var HTML = "<div><span>Test</span></div>";
var body = document.querySelector("body");

function parseHTML(htmlString){    
    var frag = document.createDocumentFragment(),
        el = document.createElement( "div" );
        el.setAttribute("id", "wrapper");
            
    el.innerHTML = HTML;
    frag.appendChild(el);
    
    return frag.querySelector("div#wrapper").children[0];
}

console.log(parseHTML(HTML));