SVGTests should not leave detached elements in the tree

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

 // [Name] SVGUseElement-svgdom-requiredFeatures.js
 // [Expected rendering result] a series of PASS messages
 
 createSVGTestCase();
 
 var useElement = createSVGElement("use");
 var defsElement = createSVGElement("defs");
 var rectElement = createSVGElement("rect");
 rectElement.setAttribute("width", "200");
 rectElement.setAttribute("height", "200");
 rectElement.setAttribute("id", "MyRect");
 useElement.setAttributeNS(xlinkNS, "xlink:href", "#MyRect");
 
 defsElement.appendChild(rectElement);
 rootSVGElement.appendChild(defsElement);
 rootSVGElement.appendChild(useElement);
 
 function repaintTest() {
     debug("Check that SVGUseElement is initially displayed");
-    shouldBeEqualToString("document.defaultView.getComputedStyle(useElement, null).display", "inline");
+    shouldHaveBBox("useElement", "200", "200");
     debug("Check that setting requiredFeatures to something invalid makes it not render");
     useElement.requiredFeatures.appendItem("http://www.w3.org/TR/SVG11/feature#BogusFeature");
-    shouldBeEqualToString("document.defaultView.getComputedStyle(useElement, null).display", "");
+    shouldHaveBBox("useElement", "0", "0");
     debug("Check that setting requiredFeatures to something valid makes it render again");
     useElement.requiredFeatures.replaceItem("http://www.w3.org/TR/SVG11/feature#Shape", 0);
-    shouldBeEqualToString("document.defaultView.getComputedStyle(useElement, null).display", "inline");
+    shouldHaveBBox("useElement", "200", "200");
     debug("Check that adding something valid to requiredFeatures keeps rendering the element");
     useElement.requiredFeatures.appendItem("http://www.w3.org/TR/SVG11/feature#Gradient");
-    shouldBeEqualToString("document.defaultView.getComputedStyle(useElement, null).display", "inline");
+    shouldHaveBBox("useElement", "200", "200");
     debug("Check that adding something invalid to requiredFeatures makes it not render");
     useElement.requiredFeatures.appendItem("http://www.w3.org/TR/SVG11/feature#BogusFeature");
-    shouldBeEqualToString("document.defaultView.getComputedStyle(useElement, null).display", "");
+    shouldHaveBBox("useElement", "0", "0");
 
     completeTest();
 }
 
 var successfullyParsed = true;