element: implement parenteNode accessors

tests/wpt/dom/Element-childElement-null-svg.svg

@@ -0,0 +1,20 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg"
+     xmlns:h="http://www.w3.org/1999/xhtml"
+     version="1.1"
+     width="100%" height="100%" viewBox="0 0 400 400">
+<title>Null test</title>
+<h:script src="/resources/testharness.js"/>
+<h:script src="/resources/testharnessreport.js"/>
+
+<text x="200" y="40" font-size="25" fill="black" text-anchor="middle">Test of firstElementChild and lastChildElement returning null</text>
+<text id="parentEl" x="200" y="70" font-size="20" fill="black" text-anchor="middle" font-weight="bold">Test</text>
+
+<h:script><![CDATA[
+test(function() {
+  var parentEl = document.getElementById("parentEl")
+  assert_equals(parentEl.firstElementChild, null)
+  assert_equals(parentEl.lastElementChild, null)
+})
+]]></h:script>
+</svg>