SVGTests should not leave detached elements in the tree

LayoutTests/svg/dynamic-updates/script-tests/SVGEllipseElement-svgdom-requiredFeatures.js

 // [Name] SVGEllipseElement-svgdom-requiredFeatures.js
 // [Expected rendering result] a series of PASS messages
 
 createSVGTestCase();
 
 var ellipseElement = createSVGElement("ellipse");
 ellipseElement.setAttribute("rx", "200");
 ellipseElement.setAttribute("ry", "200");
 
 rootSVGElement.appendChild(ellipseElement);
 
 function repaintTest() {
     debug("Check that SVGEllipseElement is initially displayed");
-    shouldBeEqualToString("document.defaultView.getComputedStyle(ellipseElement, null).display", "inline");
+    shouldHaveBBox("ellipseElement", "400", "400");
     debug("Check that setting requiredFeatures to something invalid makes it not render");
     ellipseElement.requiredFeatures.appendItem("http://www.w3.org/TR/SVG11/feature#BogusFeature");
-    shouldBeEqualToString("document.defaultView.getComputedStyle(ellipseElement, null).display", "");
+    shouldHaveBBox("ellipseElement", "0", "0");
     debug("Check that setting requiredFeatures to something valid makes it render again");
     ellipseElement.requiredFeatures.replaceItem("http://www.w3.org/TR/SVG11/feature#Shape", 0);
-    shouldBeEqualToString("document.defaultView.getComputedStyle(ellipseElement, null).display", "inline");
+    shouldHaveBBox("ellipseElement", "400", "400");
     debug("Check that adding something valid to requiredFeatures keeps rendering the element");
     ellipseElement.requiredFeatures.appendItem("http://www.w3.org/TR/SVG11/feature#Gradient");
-    shouldBeEqualToString("document.defaultView.getComputedStyle(ellipseElement, null).display", "inline");
+    shouldHaveBBox("ellipseElement", "400", "400");
     debug("Check that adding something invalid to requiredFeatures makes it not render");
     ellipseElement.requiredFeatures.appendItem("http://www.w3.org/TR/SVG11/feature#BogusFeature");
-    shouldBeEqualToString("document.defaultView.getComputedStyle(ellipseElement, null).display", "");
+    shouldHaveBBox("ellipseElement", "0", "0");
 
     completeTest();
 }
 
 var successfullyParsed = true;