diff --git a/dom/animation/test/chrome/test_restyles.html b/dom/animation/test/chrome/test_restyles.html
index 8279b197904b..df168b357ec9 100644
--- a/dom/animation/test/chrome/test_restyles.html
+++ b/dom/animation/test/chrome/test_restyles.html
@@ -66,10 +66,11 @@ SimpleTest.waitForExplicitFinish();
 var omtaEnabled = isOMTAEnabled();
 
 var isAndroid = !!navigator.userAgent.includes("Android");
+var isServo = isStyledByServo();
 var offscreenThrottlingEnabled =
   SpecialPowers.getBoolPref('dom.animations.offscreen-throttling') &&
-  !isStyledByServo(); // Bug 1303235 : For stylo we don't yet accumulate change
-                      // hints which are used for offscreen throttling.
+  !isServo; // Bug 1303235 : For stylo we don't yet accumulate change
+            // hints which are used for offscreen throttling.
 
 function add_task_if_omta_enabled(test) {
   if (!omtaEnabled) {
@@ -760,11 +761,28 @@ waitForAllPaints(function() {
     document.body.appendChild(div);
 
     markers = await observeStyling(1);
-    // Bug 1388557: We should call RequestRestyle(Layer) when an element which
-    // has running script animations is attached to a document.
-    todo_is(markers.length, 1,
-            'Bug 1388557 We should observe one restyle in the first frame ' +
-            'right after re-attaching to the document');
+    if (isServo) {
+      // In Servo, we explicitly set important_rules_change flag to the element
+      // in compute_style() during the initial normal traversal right after
+      // re-attaching which leads to invoking a SequentialTask for
+      // CascadeResults which ends up calling RequestRestyle(Standard). As a
+      // result, the animation is restyled during a second animation restyle in
+      // the first frame. If we fix the behavior when we attach an orphaned
+      // element with script animations to a document so that it requests a
+      // layer restyle (bug 1388557) before fixing important_rules_change in
+      // compute_style() so that it no longer dispatches a needless standard
+      // restyle (bug 1388560), we should add a test case that fails if we
+      // continue to unnecessarily request a standard restyle.
+      is(markers.length, 1,
+         'We should observe one restyle in the first frame right after ' +
+         're-attaching to the document');
+    } else {
+      // Bug 1388557: We should call RequestRestyle(Layer) when an element which
+      // has running script animations is attached to a document.
+      todo_is(markers.length, 1,
+              'Bug 1388557 We should observe one restyle in the first frame ' +
+              'right after re-attaching to the document');
+    }
     markers = await observeStyling(5);
     is(markers.length, 5,
        'Animation on re-attached to the document begins to update style');