diff --git a/css/selectors/media/sound-state.html b/css/selectors/media/sound-state.html
index e41e4ace2be507..fd0e8f53a4318e 100644
--- a/css/selectors/media/sound-state.html
+++ b/css/selectors/media/sound-state.html
@@ -43,5 +43,30 @@
       assert_equals(document.querySelector("video:muted"), video);
       assert_equals(document.querySelector("video:not(:muted)"), null);
     }, "Test :muted pseudo-class");
+
+    test((t) => {
+      assert_implements(CSS.supports("selector(:muted)"), ":muted is not supported");
+
+      const video = document.createElement("video");
+      document.body.appendChild(video);
+      t.add_cleanup(() => video.remove());
+
+      assert_false(video.muted);
+      assert_false(video.matches(":muted"));
+
+      video.setAttribute("muted", "");
+      assert_true(video.muted);
+      assert_true(video.matches(":muted"));
+
+      video.removeAttribute("muted");
+      assert_false(video.muted);
+      assert_false(video.matches(":muted"));
+
+      video.setAttribute("muted", "");
+      assert_true(video.muted);
+      video.muted = false;
+      assert_false(video.muted);
+      assert_false(video.matches(":muted"));
+    }, "Test :muted matches .muted for content attribute default");
   </script>
 </body>
diff --git a/html/semantics/embedded-content/media-elements/user-interface/muted.html b/html/semantics/embedded-content/media-elements/user-interface/muted.html
index eb6d2ac688d392..0bdc425750bc89 100644
--- a/html/semantics/embedded-content/media-elements/user-interface/muted.html
+++ b/html/semantics/embedded-content/media-elements/user-interface/muted.html
@@ -21,8 +21,8 @@
 }
 </script>
 
-<!-- These tests are inside <audio>/<video> so that the steps for updating the
-     muted IDL attribute cannot be delayed until the end tag is parsed. -->
+<!-- These tests are inside <audio>/<video> so that the parser-created
+     elements are available for testing before the closing tag. -->
 
 <audio id=a1>
 <script>
@@ -80,8 +80,8 @@
 </script>
 </video>
 
-<!-- Negative test to ensure that the load algorithm does not update the
-     muted IDL attribute to match the content attribute. -->
+<!-- Negative test to ensure that the load algorithm does not reset the
+     muted state. -->
 
 <video id=v3 muted></video>
 <script>
@@ -112,25 +112,22 @@
   test(function() {
     var m = document.createElement(tagName);
     m.setAttribute('muted', '');
-    assert_false(m.muted);
+    assert_true(m.muted);
   }, 'getting ' + tagName + '.muted with muted="" (script-created)');
 
   test(function() {
     var m = document.createElement(tagName);
     m.setAttribute('muted', '');
-    test_setting(m, false, true);
+    test_setting(m, true, true);
   }, 'setting ' + tagName + '.muted with muted="" (script-created)');
 
-  // Spec bug: https://www.w3.org/Bugs/Public/show_bug.cgi?id=25153
-  /*
   test(function() {
     var m = document.createElement(tagName);
     m.setAttribute('muted', '');
     m = m.cloneNode(false);
     assert_true(m.hasAttribute('muted'));
-    assert_false(m.muted);
+    assert_true(m.muted);
   }, 'getting ' + tagName + '.muted with muted="" (cloneNode-created)');
-  */
 
   test(function() {
     var div = document.createElement('div');
@@ -165,5 +162,29 @@
     var c = m.cloneNode(true);
     assert_true(c.muted);
   }, 'cloning ' + tagName + ' propagates muted (innerHTML-created)');
+
+  test(function() {
+    var m = document.createElement(tagName);
+    assert_false(m.muted);
+    m.setAttribute('muted', '');
+    assert_true(m.muted);
+    m.removeAttribute('muted');
+    assert_false(m.muted);
+  }, 'adding/removing muted attribute dynamically affects ' + tagName + '.muted');
+
+  test(function() {
+    var m = document.createElement(tagName);
+    m.muted = false;
+    m.setAttribute('muted', '');
+    assert_false(m.muted);
+  }, 'adding muted attribute has no effect on ' + tagName + '.muted after setter');
+
+  test(function() {
+    var m = document.createElement(tagName);
+    m.setAttribute('muted', '');
+    m.muted = true;
+    m.removeAttribute('muted');
+    assert_true(m.muted);
+  }, 'removing muted attribute has no effect on ' + tagName + '.muted after setter');
 });
 </script>