--- a/testing/web-platform/tests/css/css-overflow/parsing/scrollbar-gutter-valid.html
+++ b/testing/web-platform/tests/css/css-overflow/parsing/scrollbar-gutter-valid.html
@@ -7,13 +7,13 @@
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
 <script src="/css/support/parsing-testcommon.js"></script>
 
 <script>
 
     test_valid_value("scrollbar-gutter", "auto");
     test_valid_value("scrollbar-gutter", "stable");
-    test_valid_value("scrollbar-gutter", "stable mirror");
+    test_valid_value("scrollbar-gutter", "stable both-edges");
 
-    test_valid_value("scrollbar-gutter", "mirror stable", "stable mirror");
+    test_valid_value("scrollbar-gutter", "both-edges stable", "stable both-edges");
 
 </script>

--- a/testing/web-platform/tests/css/css-overflow/scrollbar-gutter-001.html
+++ b/testing/web-platform/tests/css/css-overflow/scrollbar-gutter-001.html
@@ -38,17 +38,17 @@
     scrollbar-gutter: auto;
   }
 
   .sg_stable {
     scrollbar-gutter: stable;
   }
 
   .sg_stable_mirror {
-    scrollbar-gutter: stable mirror;
+    scrollbar-gutter: stable both-edges;
   }
 
   /* overflow */
   .container.ov_auto {
     overflow-y: auto;
   }
 
   .container.ov_scroll {
@@ -110,33 +110,33 @@
 
     <div class="container ov_clip sg_stable" id="container_clip_stable">
       <div class="content" id="content_clip_stable">clip/stable</div>
     </div>
   </div>
 
   <div class="line">
     <div class="container ov_auto sg_stable_mirror" id="container_auto_stable_mirror">
-      <div class="content" id="content_auto_stable_mirror">auto/stable mirror</div>
+      <div class="content" id="content_auto_stable_mirror">auto/stable both-edges</div>
     </div>
 
     <div class="container ov_scroll sg_stable_mirror" id="container_scroll_stable_mirror">
-      <div class="content" id="content_scroll_stable_mirror">scroll/stable mirror</div>
+      <div class="content" id="content_scroll_stable_mirror">scroll/stable both-edges</div>
     </div>
 
     <div class="container ov_visible sg_stable_mirror" id="container_visible_stable_mirror">
-      <div class="content" id="content_visible_stable_mirror">visible/stable mirror</div>
+      <div class="content" id="content_visible_stable_mirror">visible/stable both-edges</div>
     </div>
 
     <div class="container ov_hidden sg_stable_mirror" id="container_hidden_stable_mirror">
-      <div class="content" id="content_hidden_stable_mirror">hidden/stable mirror</div>
+      <div class="content" id="content_hidden_stable_mirror">hidden/stable both-edges</div>
     </div>
 
     <div class="container ov_clip sg_stable_mirror" id="container_clip_stable_mirror">
-      <div class="content" id="content_clip_stable_mirror">clip/stable mirror</div>
+      <div class="content" id="content_clip_stable_mirror">clip/stable both-edges</div>
     </div>
   </div>
 
   <script type="text/javascript">
     setup({ explicit_done: true });
 
     // scrollbar-gutter: auto
 
@@ -207,55 +207,55 @@
 
     test(function () {
       let container = document.getElementById('container_clip_stable');
       let content = document.getElementById('content_clip_stable');
       assert_equals(container.offsetWidth, content.offsetWidth, "content width");
       assert_equals(container.offsetLeft, content.offsetLeft, "content position");
     }, "overflow clip, scrollbar-gutter stable");
 
-    // scrollbar-gutter: stable mirror
+    // scrollbar-gutter: stable both-edges
 
     test(function () {
       let container = document.getElementById('container_auto_stable_mirror');
       let content = document.getElementById('content_auto_stable_mirror');
       assert_greater_than(container.offsetWidth, content.offsetWidth, "content width");
       assert_less_than(container.offsetLeft, content.offsetLeft, "content position");
       let reference = document.getElementById('content_auto_stable');
       assert_less_than(content.offsetWidth, reference.offsetWidth, "compare with \"stable\"");
-    }, "overflow auto, scrollbar-gutter stable mirror");
+    }, "overflow auto, scrollbar-gutter stable both-edges");
 
     test(function () {
       let container = document.getElementById('container_scroll_stable_mirror');
       let content = document.getElementById('content_scroll_stable_mirror');
       assert_greater_than(container.offsetWidth, content.offsetWidth, "content width");
       assert_less_than(container.offsetLeft, content.offsetLeft, "content position");
       let reference = document.getElementById('content_auto_stable');
       assert_less_than(content.offsetWidth, reference.offsetWidth, "compare with \"stable\"");
-    }, "overflow scroll, scrollbar-gutter stable mirror");
+    }, "overflow scroll, scrollbar-gutter stable both-edges");
 
     test(function () {
       let container = document.getElementById('container_visible_stable_mirror');
       let content = document.getElementById('content_visible_stable_mirror');
       assert_equals(container.offsetWidth, content.offsetWidth, "content width");
       assert_equals(container.offsetLeft, content.offsetLeft, "content position");
-    }, "overflow visible, scrollbar-gutter stable mirror");
+    }, "overflow visible, scrollbar-gutter stable both-edges");
 
     test(function () {
       let container = document.getElementById('container_hidden_stable_mirror');
       let content = document.getElementById('content_hidden_stable_mirror');
       assert_greater_than(container.offsetWidth, content.offsetWidth, "content width");
       assert_less_than(container.offsetLeft, content.offsetLeft, "content position");
       let reference = document.getElementById('content_auto_stable');
       assert_less_than(content.offsetWidth, reference.offsetWidth, "compare with \"stable\"");
-    }, "overflow hidden, scrollbar-gutter stable mirror");
+    }, "overflow hidden, scrollbar-gutter stable both-edges");
 
     test(function () {
       let container = document.getElementById('container_clip_stable_mirror');
       let content = document.getElementById('content_clip_stable_mirror');
       assert_equals(container.offsetWidth, content.offsetWidth, "content width");
       assert_equals(container.offsetLeft, content.offsetLeft, "content position");
-    }, "overflow clip, scrollbar-gutter stable mirror");
+    }, "overflow clip, scrollbar-gutter stable both-edges");
 
     done();
 
   </script>
 </body>

--- a/testing/web-platform/tests/css/css-overflow/scrollbar-gutter-rtl-001.html
+++ b/testing/web-platform/tests/css/css-overflow/scrollbar-gutter-rtl-001.html
@@ -38,17 +38,17 @@
     scrollbar-gutter: auto;
   }
 
   .sg_stable {
     scrollbar-gutter: stable;
   }
 
   .sg_stable_mirror {
-    scrollbar-gutter: stable mirror;
+    scrollbar-gutter: stable both-edges;
   }
 
   /* overflow */
   .container.ov_auto {
     overflow-y: auto;
   }
 
   .container.ov_scroll {
@@ -110,33 +110,33 @@
 
     <div class="container ov_clip sg_stable" id="container_clip_stable">
       <div class="content" id="content_clip_stable">clip/stable</div>
     </div>
   </div>
 
   <div class="line">
     <div class="container ov_auto sg_stable_mirror" id="container_auto_stable_mirror">
-      <div class="content" id="content_auto_stable_mirror">auto/stable mirror</div>
+      <div class="content" id="content_auto_stable_mirror">auto/stable both-edges</div>
     </div>
 
     <div class="container ov_scroll sg_stable_mirror" id="container_scroll_stable_mirror">
-      <div class="content" id="content_scroll_stable_mirror">scroll/stable mirror</div>
+      <div class="content" id="content_scroll_stable_mirror">scroll/stable both-edges</div>
     </div>
 
     <div class="container ov_visible sg_stable_mirror" id="container_visible_stable_mirror">
-      <div class="content" id="content_visible_stable_mirror">visible/stable mirror</div>
+      <div class="content" id="content_visible_stable_mirror">visible/stable both-edges</div>
     </div>
 
     <div class="container ov_hidden sg_stable_mirror" id="container_hidden_stable_mirror">
-      <div class="content" id="content_hidden_stable_mirror">hidden/stable mirror</div>
+      <div class="content" id="content_hidden_stable_mirror">hidden/stable both-edges</div>
     </div>
 
     <div class="container ov_clip sg_stable_mirror" id="container_clip_stable_mirror">
-      <div class="content" id="content_clip_stable_mirror">clip/stable mirror</div>
+      <div class="content" id="content_clip_stable_mirror">clip/stable both-edges</div>
     </div>
   </div>
 
   <script type="text/javascript">
     setup({ explicit_done: true });
 
     // scrollbar-gutter: auto
 
@@ -207,55 +207,55 @@
 
     test(function () {
       let container = document.getElementById('container_clip_stable');
       let content = document.getElementById('content_clip_stable');
       assert_equals(container.offsetWidth, content.offsetWidth, "content width");
       assert_equals(container.offsetLeft, content.offsetLeft, "content position");
     }, "overflow clip, scrollbar-gutter stable");
 
-    // scrollbar-gutter: stable mirror
+    // scrollbar-gutter: stable both-edges
 
     test(function () {
       let container = document.getElementById('container_auto_stable_mirror');
       let content = document.getElementById('content_auto_stable_mirror');
       assert_greater_than(container.offsetWidth, content.offsetWidth, "content width");
       assert_less_than(container.offsetLeft, content.offsetLeft, "content position");
       let reference = document.getElementById('content_auto_stable');
       assert_less_than(content.offsetWidth, reference.offsetWidth, "compare with \"stable\"");
-    }, "overflow auto, scrollbar-gutter stable mirror");
+    }, "overflow auto, scrollbar-gutter stable both-edges");
 
     test(function () {
       let container = document.getElementById('container_scroll_stable_mirror');
       let content = document.getElementById('content_scroll_stable_mirror');
       assert_greater_than(container.offsetWidth, content.offsetWidth, "content width");
       assert_less_than(container.offsetLeft, content.offsetLeft, "content position");
       let reference = document.getElementById('content_scroll_stable');
       assert_less_than(content.offsetWidth, reference.offsetWidth, "compare with \"stable\"");
-    }, "overflow scroll, scrollbar-gutter stable mirror");
+    }, "overflow scroll, scrollbar-gutter stable both-edges");
 
     test(function () {
       let container = document.getElementById('container_visible_stable_mirror');
       let content = document.getElementById('content_visible_stable_mirror');
       assert_equals(container.offsetWidth, content.offsetWidth, "content width");
       assert_equals(container.offsetLeft, content.offsetLeft, "content position");
-    }, "overflow visible, scrollbar-gutter stable mirror");
+    }, "overflow visible, scrollbar-gutter stable both-edges");
 
     test(function () {
       let container = document.getElementById('container_hidden_stable_mirror');
       let content = document.getElementById('content_hidden_stable_mirror');
       assert_greater_than(container.offsetWidth, content.offsetWidth, "content width");
       assert_less_than(container.offsetLeft, content.offsetLeft, "content position");
       let reference = document.getElementById('content_auto_stable');
       assert_less_than(content.offsetWidth, reference.offsetWidth, "compare with \"stable\"");
-    }, "overflow hidden, scrollbar-gutter stable mirror");
+    }, "overflow hidden, scrollbar-gutter stable both-edges");
 
     test(function () {
       let container = document.getElementById('container_clip_stable_mirror');
       let content = document.getElementById('content_clip_stable_mirror');
       assert_equals(container.offsetWidth, content.offsetWidth, "content width");
       assert_equals(container.offsetLeft, content.offsetLeft, "content position");
-    }, "overflow clip, scrollbar-gutter stable mirror");
+    }, "overflow clip, scrollbar-gutter stable both-edges");
 
     done();
 
   </script>
 </body>

--- a/testing/web-platform/tests/css/css-overflow/scrollbar-gutter-vertical-lr-001.html
+++ b/testing/web-platform/tests/css/css-overflow/scrollbar-gutter-vertical-lr-001.html
@@ -36,17 +36,17 @@
     scrollbar-gutter: auto;
   }
 
   .sg_stable {
     scrollbar-gutter: stable;
   }
 
   .sg_stable_mirror {
-    scrollbar-gutter: stable mirror;
+    scrollbar-gutter: stable both-edges;
   }
 
   /* overflow */
   .container.ov_auto {
     overflow-x: auto;
   }
 
   .container.ov_scroll {
@@ -108,33 +108,33 @@
 
     <div class="container ov_clip sg_stable" id="container_clip_stable">
       <div class="content" id="content_clip_stable">clip/stable</div>
     </div>
   </div>
 
   <div class="line">
     <div class="container ov_auto sg_stable_mirror" id="container_auto_stable_mirror">
-      <div class="content" id="content_auto_stable_mirror">auto/stable mirror</div>
+      <div class="content" id="content_auto_stable_mirror">auto/stable both-edges</div>
     </div>
 
     <div class="container ov_scroll sg_stable_mirror" id="container_scroll_stable_mirror">
-      <div class="content" id="content_scroll_stable_mirror">scroll/stable mirror</div>
+      <div class="content" id="content_scroll_stable_mirror">scroll/stable both-edges</div>
     </div>
 
     <div class="container ov_visible sg_stable_mirror" id="container_visible_stable_mirror">
-      <div class="content" id="content_visible_stable_mirror">visible/stable mirror</div>
+      <div class="content" id="content_visible_stable_mirror">visible/stable both-edges</div>
     </div>
 
     <div class="container ov_hidden sg_stable_mirror" id="container_hidden_stable_mirror">
-      <div class="content" id="content_hidden_stable_mirror">hidden/stable mirror</div>
+      <div class="content" id="content_hidden_stable_mirror">hidden/stable both-edges</div>
     </div>
 
     <div class="container ov_clip sg_stable_mirror" id="container_clip_stable_mirror">
-      <div class="content" id="content_clip_stable_mirror">clip/stable mirror</div>
+      <div class="content" id="content_clip_stable_mirror">clip/stable both-edges</div>
     </div>
   </div>
 
   <script type="text/javascript">
     setup({ explicit_done: true });
 
     // scrollbar-gutter: auto
 
@@ -205,55 +205,55 @@
 
     test(function () {
       let container = document.getElementById('container_clip_stable');
       let content = document.getElementById('content_clip_stable');
       assert_equals(container.offsetHeight, content.offsetHeight, "content height");
       assert_equals(container.offsetTop, content.offsetTop, "content position");
     }, "overflow clip, scrollbar-gutter stable");
 
-    // scrollbar-gutter: stable mirror
+    // scrollbar-gutter: stable both-edges
 
     test(function () {
       let container = document.getElementById('container_auto_stable_mirror');
       let content = document.getElementById('content_auto_stable_mirror');
       assert_greater_than(container.offsetHeight, content.offsetHeight, "content height");
       assert_less_than(container.offsetTop, content.offsetTop, "content position");
       let reference = document.getElementById('content_auto_stable');
       assert_less_than(content.offsetHeight, reference.offsetHeight, "compare with \"stable\"");
-    }, "overflow auto, scrollbar-gutter stable mirror");
+    }, "overflow auto, scrollbar-gutter stable both-edges");
 
     test(function () {
       let container = document.getElementById('container_scroll_stable_mirror');
       let content = document.getElementById('content_scroll_stable_mirror');
       assert_greater_than(container.offsetHeight, content.offsetHeight, "content height");
       assert_less_than(container.offsetTop, content.offsetTop, "content position");
       let reference = document.getElementById('content_auto_stable');
       assert_less_than(content.offsetHeight, reference.offsetHeight, "compare with \"stable\"");
-    }, "overflow scroll, scrollbar-gutter stable mirror");
+    }, "overflow scroll, scrollbar-gutter stable both-edges");
 
     test(function () {
       let container = document.getElementById('container_visible_stable_mirror');
       let content = document.getElementById('content_visible_stable_mirror');
       assert_equals(container.offsetHeight, content.offsetHeight, "content height");
       assert_equals(container.offsetTop, content.offsetTop, "content position");
-    }, "overflow visible, scrollbar-gutter stable mirror");
+    }, "overflow visible, scrollbar-gutter stable both-edges");
 
     test(function () {
       let container = document.getElementById('container_hidden_stable_mirror');
       let content = document.getElementById('content_hidden_stable_mirror');
       assert_greater_than(container.offsetHeight, content.offsetHeight, "content height");
       assert_less_than(container.offsetTop, content.offsetTop, "content position");
       let reference = document.getElementById('content_auto_stable');
       assert_less_than(content.offsetHeight, reference.offsetHeight, "compare with \"stable\"");
-    }, "overflow hidden, scrollbar-gutter stable mirror");
+    }, "overflow hidden, scrollbar-gutter stable both-edges");
 
     test(function () {
       let container = document.getElementById('container_clip_stable_mirror');
       let content = document.getElementById('content_clip_stable_mirror');
       assert_equals(container.offsetHeight, content.offsetHeight, "content height");
       assert_equals(container.offsetTop, content.offsetTop, "content position");
-    }, "overflow clip, scrollbar-gutter stable mirror");
+    }, "overflow clip, scrollbar-gutter stable both-edges");
 
     done();
 
   </script>
 </body>

--- a/testing/web-platform/tests/css/css-overflow/scrollbar-gutter-vertical-rl-001.html
+++ b/testing/web-platform/tests/css/css-overflow/scrollbar-gutter-vertical-rl-001.html
@@ -36,17 +36,17 @@
     scrollbar-gutter: auto;
   }
 
   .sg_stable {
     scrollbar-gutter: stable;
   }
 
   .sg_stable_mirror {
-    scrollbar-gutter: stable mirror;
+    scrollbar-gutter: stable both-edges;
   }
 
   /* overflow */
   .container.ov_auto {
     overflow-x: auto;
   }
 
   .container.ov_scroll {
@@ -108,33 +108,33 @@
 
     <div class="container ov_clip sg_stable" id="container_clip_stable">
       <div class="content" id="content_clip_stable">clip/stable</div>
     </div>
   </div>
 
   <div class="line">
     <div class="container ov_auto sg_stable_mirror" id="container_auto_stable_mirror">
-      <div class="content" id="content_auto_stable_mirror">auto/stable mirror</div>
+      <div class="content" id="content_auto_stable_mirror">auto/stable both-edges</div>
     </div>
 
     <div class="container ov_scroll sg_stable_mirror" id="container_scroll_stable_mirror">
-      <div class="content" id="content_scroll_stable_mirror">scroll/stable mirror</div>
+      <div class="content" id="content_scroll_stable_mirror">scroll/stable both-edges</div>
     </div>
 
     <div class="container ov_visible sg_stable_mirror" id="container_visible_stable_mirror">
-      <div class="content" id="content_visible_stable_mirror">visible/stable mirror</div>
+      <div class="content" id="content_visible_stable_mirror">visible/stable both-edges</div>
     </div>
 
     <div class="container ov_hidden sg_stable_mirror" id="container_hidden_stable_mirror">
-      <div class="content" id="content_hidden_stable_mirror">hidden/stable mirror</div>
+      <div class="content" id="content_hidden_stable_mirror">hidden/stable both-edges</div>
     </div>
 
     <div class="container ov_clip sg_stable_mirror" id="container_clip_stable_mirror">
-      <div class="content" id="content_clip_stable_mirror">clip/stable mirror</div>
+      <div class="content" id="content_clip_stable_mirror">clip/stable both-edges</div>
     </div>
   </div>
 
   <script type="text/javascript">
 
     setup({ explicit_done: true });
 
     // scrollbar-gutter: auto
@@ -206,55 +206,55 @@
 
     test(function () {
       let container = document.getElementById('container_clip_stable');
       let content = document.getElementById('content_clip_stable');
       assert_equals(container.offsetHeight, content.offsetHeight, "content height");
       assert_equals(container.offsetTop, content.offsetTop, "content position");
     }, "overflow clip, scrollbar-gutter stable");
 
-    // scrollbar-gutter: stable mirror
+    // scrollbar-gutter: stable both-edges
 
     test(function () {
       let container = document.getElementById('container_auto_stable_mirror');
       let content = document.getElementById('content_auto_stable_mirror');
       assert_greater_than(container.offsetHeight, content.offsetHeight, "content height");
       assert_less_than(container.offsetTop, content.offsetTop, "content position");
       let reference = document.getElementById('content_auto_stable');
       assert_less_than(content.offsetHeight, reference.offsetHeight, "compare with \"stable\"");
-    }, "overflow auto, scrollbar-gutter stable mirror");
+    }, "overflow auto, scrollbar-gutter stable both-edges");
 
     test(function () {
       let container = document.getElementById('container_scroll_stable_mirror');
       let content = document.getElementById('content_scroll_stable_mirror');
       assert_greater_than(container.offsetHeight, content.offsetHeight, "content height");
       assert_less_than(container.offsetTop, content.offsetTop, "content position");
       let reference = document.getElementById('content_auto_stable');
       assert_less_than(content.offsetHeight, reference.offsetHeight, "compare with \"stable\"");
-    }, "overflow scroll, scrollbar-gutter stable mirror");
+    }, "overflow scroll, scrollbar-gutter stable both-edges");
 
     test(function () {
       let container = document.getElementById('container_visible_stable_mirror');
       let content = document.getElementById('content_visible_stable_mirror');
       assert_equals(container.offsetHeight, content.offsetHeight, "content height");
       assert_equals(container.offsetTop, content.offsetTop, "content position");
-    }, "overflow visible, scrollbar-gutter stable mirror");
+    }, "overflow visible, scrollbar-gutter stable both-edges");
 
     test(function () {
       let container = document.getElementById('container_hidden_stable_mirror');
       let content = document.getElementById('content_hidden_stable_mirror');
       assert_greater_than(container.offsetHeight, content.offsetHeight, "content height");
       assert_less_than(container.offsetTop, content.offsetTop, "content position");
       let reference = document.getElementById('content_auto_stable');
       assert_less_than(content.offsetHeight, reference.offsetHeight, "compare with \"stable\"");
-    }, "overflow hidden, scrollbar-gutter stable mirror");
+    }, "overflow hidden, scrollbar-gutter stable both-edges");
 
     test(function () {
       let container = document.getElementById('container_clip_stable_mirror');
       let content = document.getElementById('content_clip_stable_mirror');
       assert_equals(container.offsetHeight, content.offsetHeight, "content height");
       assert_equals(container.offsetTop, content.offsetTop, "content position");
-    }, "overflow clip, scrollbar-gutter stable mirror");
+    }, "overflow clip, scrollbar-gutter stable both-edges");
 
     done();
 
   </script>
 </body>

--- a/testing/web-platform/tests/css/css-typed-om/the-stylepropertymap/properties/scrollbar-gutter.html
+++ b/testing/web-platform/tests/css/css-typed-om/the-stylepropertymap/properties/scrollbar-gutter.html
@@ -13,12 +13,12 @@
 <script>
 'use strict';
 
 runPropertyTests('scrollbar-gutter', [
     { syntax: 'auto' },
     { syntax: 'stable' }
 ]);
 runUnsupportedPropertyTests('scrollbar-gutter', [
-    'stable mirror'
+  'stable both-edges'
 ]);
 
 </script>