--- a/dom/animation/test/chrome/test_animation_properties.html
+++ b/dom/animation/test/chrome/test_animation_properties.html
@@ -19,21 +19,16 @@
 }
 div {
   font-size: 10px; /* For calculating em-based units */
 }
 </style>
 <script>
 'use strict';
 
-// Stylo matches the spec more closely wrt serialization of specified calc().
-//
-// See https://github.com/w3c/csswg-drafts/issues/1731
-const isStylo = SpecialPowers.DOMWindowUtils.isStyledByServo;
-
 var gTests = [
 
   // ---------------------------------------------------------------------
   //
   // Tests for property-indexed specifications
   //
   // ---------------------------------------------------------------------
 
@@ -566,17 +561,17 @@ var gTests = [
     expected: [ { property: 'left',
                   values: [ valueFormat(0, '100px', 'replace', 'linear'),
                             valueFormat(1, '200px', 'replace') ] } ]
   },
   { desc:     'calc() expressions are resolved to the equivalent units',
     frames:   { left: ['calc(10em + 10px)', 'calc(10em + 10%)'] },
     expected: [ { property: 'left',
                   values: [ valueFormat(0, 'calc(110px)', 'replace', 'linear'),
-                            valueFormat(1, isStylo ? 'calc(10% + 100px)' : 'calc(100px + 10%)', 'replace') ] } ]
+                            valueFormat(1, 'calc(10% + 100px)', 'replace') ] } ]
   },
 
   // ---------------------------------------------------------------------
   //
   // Tests for CSS variable handling conversion
   //
   // ---------------------------------------------------------------------
 
@@ -585,17 +580,17 @@ var gTests = [
     expected: [ { property: 'left',
                   values: [ valueFormat(0, '10px', 'replace', 'linear'),
                             valueFormat(1, '100px', 'replace') ] } ]
   },
   { desc:     'CSS variables in calc() expressions are resolved',
     frames:   { left: ['10px', 'calc(var(--var-100px) / 2 - 10%)'] },
     expected: [ { property: 'left',
                   values: [ valueFormat(0, '10px', 'replace', 'linear'),
-                            valueFormat(1, isStylo ? 'calc(-10% + 50px)' : 'calc(50px + -10%)', 'replace') ] } ]
+                            valueFormat(1, 'calc(-10% + 50px)', 'replace') ] } ]
   },
   { desc:     'CSS variables in shorthands are resolved to their corresponding'
               + ' values',
     frames:   { margin: ['10px', 'var(--var-100px-200px)'] },
     expected: [ { property: 'margin-top',
                   values: [ valueFormat(0, '10px', 'replace', 'linear'),
                             valueFormat(1, '100px', 'replace') ] },
                 { property: 'margin-right',

--- a/dom/animation/test/mozilla/file_restyles.html
+++ b/dom/animation/test/mozilla/file_restyles.html
@@ -159,17 +159,16 @@ function tweakExpectedRestyleCount(aAnim
     return aExpectedRestyleCount + 1;
   }
   return aExpectedRestyleCount;
 }
 
 var omtaEnabled = isOMTAEnabled();
 
 var isAndroid = !!navigator.userAgent.includes("Android");
-var isServo = isStyledByServo();
 var hasConformantPromiseHandling;
 
 function add_task_if_omta_enabled(test) {
   if (!omtaEnabled) {
     info(test.name + " is skipped because OMTA is disabled");
     return;
   }
   add_task(test);
@@ -1265,34 +1264,22 @@ waitForAllPaints(() => {
   add_task(async function necessary_update_should_be_invoked() {
     var div = addDiv(null, { style: 'animation: background-color 100s' });
     var animation = div.getAnimations()[0];
     await animation.ready;
     await waitForAnimationFrames(5);
     // Apply another animation style
     div.style.animation = 'background-color 110s';
     var markers = await observeStyling(1);
-    if (isServo) {
-      // There should be two restyles.
-      // 1) Animation-only restyle for before applying the new animation style
-      // 2) Animation-only restyle for after applying the new animation style
-      is(markers.length, 2,
-         'Applying animation style with different duration '  +
-         'should restyle twice');
-    } else {
-      // There should be three restyles.
-      // 1) Animation-only restyle for before applying the new animation style
-      // 2) Restyle for applying the new animation style
-      //    Note: In gecko styling for animations is not separated.
-      // 3) Restyle triggered by updating an existing animation (specifically
-      //    the animation-duration)
-      is(markers.length, 3,
-         'Applying animation style with different duration '  +
-         'should restyles three times');
-    }
+    // There should be two restyles.
+    // 1) Animation-only restyle for before applying the new animation style
+    // 2) Animation-only restyle for after applying the new animation style
+    is(markers.length, 2,
+       'Applying animation style with different duration '  +
+       'should restyle twice');
     await ensureElementRemoval(div);
   });
 
   add_task_if_omta_enabled(
     async function changing_cascading_result_for_main_thread_animation() {
       var div = addDiv(null, { style: 'background-color: blue' });
       var animation = div.animate({ opacity: [0, 1],
                                     backgroundColor: ['green', 'red'] },
@@ -1723,25 +1710,19 @@ waitForAllPaints(() => {
       var animation = div.animate({ opacity: [0, 1] }, 100 * MS_PER_SEC);
 
       await animation.ready;
       ok(SpecialPowers.wrap(animation).isRunningOnCompositor,
          'The opacity animation is running on the compositor');
 
       div.style.setProperty('color', 'blue', '');
       var markers = await observeStyling(5);
-      if (isServo) {
-        is(markers.length, 0,
-           'The opacity animation keeps running on the compositor when ' +
-           'color style is changed');
-      } else {
-        todo_is(markers.length, 0,
-                'Bug 1307341 The opacity animation keeps running on the ' +
-                'compositor when color style is changed');
-      }
+      is(markers.length, 0,
+         'The opacity animation keeps running on the compositor when ' +
+         'color style is changed');
       await ensureElementRemoval(div);
     }
   );
 
   add_task(
     async function no_overflow_transform_animations_in_scrollable_element() {
       await SpecialPowers.pushPrefEnv({ set: [["ui.showHideScrollbars", 1]] });
 
@@ -1789,26 +1770,19 @@ waitForAllPaints(() => {
     await animation.ready;
     ok(SpecialPowers.wrap(animation).isRunningOnCompositor);
 
     var markers = observeAnimSyncStyling(() => {
       div.style.animationDuration = '0s';
       is(div.getAnimations().length, 0, 'There should be no animation');
     });
 
-    if (isServo) {
-      is(markers.length, 1, // For discarding the throttled animation.
-         'Element.getAnimations() should flush throttled animation style so ' +
-         'that the throttled animation is discarded');
-    } else {
-      is(markers.length, 2, // One is done through UpdateOnlyAnimationStyles(),
-                            // the other is for discarding the animation.
-         'Element.getAnimations() should flush throttled animation style that ' +
-         'the throttled animation is discarded');
-    }
+    is(markers.length, 1, // For discarding the throttled animation.
+       'Element.getAnimations() should flush throttled animation style so ' +
+       'that the throttled animation is discarded');
 
     await ensureElementRemoval(div);
   });
 
   add_task(
     async function no_restyling_for_throttled_animation_on_querying_play_state() {
       var div = addDiv(null, { style: 'animation: opacity 100s' });
       var animation = div.getAnimations()[0];

--- a/dom/animation/test/mozilla/test_restyling_xhr_doc.html
+++ b/dom/animation/test/mozilla/test_restyling_xhr_doc.html
@@ -79,39 +79,28 @@ promise_test(t => {
     // Gecko currently skips applying animation styles to elements in documents
     // without browsing contexts.
     assert_not_equals(getComputedStyle(div).opacity, '0',
                       'Style should NOT be updated');
   });
 }, 'Forcing an animation targetting an element in a document without a'
     + ' browsing context to play does not cause style to update');
 
-// Following is an additional Gecko-specific test to confirm the behavior
-// when we have an element with an animation restyle and then move it to
-// a document without a pres shell.
-
 promise_test(t => {
   let anim;
   return getXHRDoc(t).then(xhrdoc => {
     const div = addDiv(t);
     anim = div.animate({ opacity: [ 0, 1 ] }, 1000);
     assert_equals(getComputedStyle(div).opacity, '0',
                   'Style should be updated');
     // Trigger an animation restyle to be queued
     anim.currentTime = 0.1;
     // Adopt node into XHR doc
     xhrdoc.body.appendChild(div);
     // We should skip applying animation styles to elements in documents
     // without a pres shell.
-    //
-    // The Gecko style backend, however, does not do this. Since we expect the
-    // Gecko style backend to be obsolete in the near future, we only perform
-    // this check when the Servo backend is in use.
-    let isServo = SpecialPowers.DOMWindowUtils.isStyledByServo;
-    if (isServo) {
-      assert_equals(getComputedStyle(div).opacity, '1',
-                    'Style should NOT be updated');
-    }
+    assert_equals(getComputedStyle(div).opacity, '1',
+                  'Style should NOT be updated');
   });
 }, 'Moving an element with a pending animation restyle to a document without'
    + ' a browsing context resets animation style');
 
 </script>

--- a/dom/animation/test/mozilla/test_underlying_discrete_value.html
+++ b/dom/animation/test/mozilla/test_underlying_discrete_value.html
@@ -3,18 +3,16 @@
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
 <script src="../testcommon.js"></script>
 <body>
 <div id="log"></div>
 <script>
 "use strict";
 
-const isGecko = !isStyledByServo();
-
 // Tests that we correctly extract the underlying value when the animation
 // type is 'discrete'.
 const discreteTests = [
   {
     stylesheet: {
       "@keyframes keyframes":
       "from { align-content: flex-start; } to { align-content: flex-end; } "
     },
@@ -28,36 +26,26 @@ const discreteTests = [
     stylesheet: {
       "@keyframes keyframes": "from { align-content: flex-start; }"
     },
     expectedKeyframes: [
       { computedOffset: 0, alignContent: "flex-start" },
       { computedOffset: 1, alignContent: "normal" }
     ],
     explanation: "Test for 0% keyframe only",
-    // The value of 100% should be 'normal',
-    // but we are not supporting underlying value for Gecko.
-    // https://bugzilla.mozilla.org/show_bug.cgi?id=1295401
-    // So, we skip this test case.
-    skip: isGecko
   },
   {
     stylesheet: {
       "@keyframes keyframes": "to { align-content: flex-end; }"
     },
     expectedKeyframes: [
       { computedOffset: 0, alignContent: "normal" },
       { computedOffset: 1, alignContent: "flex-end" }
     ],
     explanation: "Test for 100% keyframe only",
-    // The value of 0% should be 'normal',
-    // but we are not supporting underlying value for Gecko.
-    // https://bugzilla.mozilla.org/show_bug.cgi?id=1295401
-    // So, we skip this test case.
-    skip: isGecko
   },
   {
     stylesheet: {
       "@keyframes keyframes": "50% { align-content: center; }",
       "#target": "align-content: space-between;"
     },
     expectedKeyframes: [
       { computedOffset: 0, alignContent: "space-between" },
@@ -89,21 +77,16 @@ const discreteTests = [
     },
     expectedKeyframes: [
       { computedOffset: 0, alignContent: "normal" },
       { computedOffset: 0.5, alignContent: "center" },
       { computedOffset: 1, alignContent: "normal" }
     ],
     explanation: "Test for no 0%/100% keyframes " +
                  "and 'inherit' specified on target element",
-    // The value of 0%/100% should be 'normal',
-    // but we are not supporting underlying value for Gecko.
-    // https://bugzilla.mozilla.org/show_bug.cgi?id=1295401
-    // So, we skip this test case.
-    skip: isGecko
   },
   {
     stylesheet: {
       "@keyframes keyframes": "50% { align-content: center; }",
       ".target": "align-content: space-between;"
     },
     attributes: {
       class: "target"

--- a/dom/animation/test/testcommon.js
+++ b/dom/animation/test/testcommon.js
@@ -359,23 +359,16 @@ 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) {