--- a/dom/animation/test/testcommon.js
+++ b/dom/animation/test/testcommon.js
@@ -315,16 +315,23 @@ function useTestRefreshMode(t) {
  */
 function isOMTAEnabled() {
   const OMTAPrefKey = 'layers.offmainthreadcomposition.async-animations';
   return SpecialPowers.DOMWindowUtils.layerManagerRemote &&
          SpecialPowers.getBoolPref(OMTAPrefKey);
 }
 
 /**
+ * Returns true if the document is styled by servo.
+ */
+function isStyledByServo() {
+  return SpecialPowers.DOMWindowUtils.isStyledByServo;
+}
+
+/**
  * Append an SVG element to the target element.
  *
  * @param target The element which want to append.
  * @param attrs  A array object with attribute name and values to set on
  *               the SVG element.
  * @return An SVG outer element.
  */
 function addSVGElement(target, tag, attrs) {