[VarDumper] fixed HtmlDumper to target specific the head tag

src/Symfony/Component/VarDumper/Dumper/HtmlDumper.php

@@ -129,7 +129,7 @@ protected function getDumpHeader()
         e.addEventListener(n, cb, false);
     };
 
-doc.documentElement.firstChild.appendChild(refStyle);
+(doc.documentElement.firstElementChild || doc.documentElement.children[0]).appendChild(refStyle);
 
 if (!doc.addEventListener) {
     addEventListener = function (element, eventName, callback) {