diff --git a/src/browser/Frame.zig b/src/browser/Frame.zig
index 54dc7a4c7..6adba3bbf 100644
--- a/src/browser/Frame.zig
+++ b/src/browser/Frame.zig
@@ -2364,6 +2364,9 @@ pub fn dupeSSO(self: *Frame, value: []const u8) !String {
 
 const RemoveNodeOpts = struct {
     will_be_reconnected: bool,
+    // Set to false when the caller queues its own combined mutation record
+    // (e.g. replaceChildren's single "replace all" record).
+    notify_observers: bool = true,
 };
 pub fn removeNode(self: *Frame, parent: *Node, child: *Node, opts: RemoveNodeOpts) void {
     // Capture siblings before removing
@@ -2398,7 +2401,7 @@ pub fn removeNode(self: *Frame, parent: *Node, child: *Node, opts: RemoveNodeOpt
 
     slotting.removalSteps(parent, child, self);
 
-    if (observers.hasMutationObservers(self)) {
+    if (opts.notify_observers and observers.hasMutationObservers(self)) {
         const removed = [_]*Node{child};
         observers.notifyChildListChange(self, parent, &.{}, &removed, previous_sibling, next_sibling);
     }
@@ -2497,6 +2500,9 @@ const InsertNodeRelative = union(enum) {
 const InsertNodeOpts = struct {
     child_already_connected: bool = false,
     adopting_to_new_document: bool = false,
+    // Set to false when the caller queues its own combined mutation record
+    // (e.g. replaceChildren's single "replace all" record).
+    notify_observers: bool = true,
 };
 pub fn insertNodeRelative(self: *Frame, parent: *Node, child: *Node, relative: InsertNodeRelative, opts: InsertNodeOpts) !void {
     return self._insertNodeRelative(false, parent, child, relative, opts);
@@ -2590,7 +2596,9 @@ pub fn _insertNodeRelative(self: *Frame, comptime from_parser: bool, parent: *No
         }
     }
 
-    self.notifyChildInserted(parent, child);
+    if (opts.notify_observers) {
+        self.notifyChildInserted(parent, child);
+    }
 
     if (opts.child_already_connected and !opts.adopting_to_new_document) {
         // The child is already connected in the same document, we don't have to reconnect it.
diff --git a/src/browser/webapi/Node.zig b/src/browser/webapi/Node.zig
index 8cd407988..bbe5d1f9e 100644
--- a/src/browser/webapi/Node.zig
+++ b/src/browser/webapi/Node.zig
@@ -1282,10 +1282,19 @@ pub fn replaceChildren(self: *Node, nodes: []const NodeOrText, frame: *Frame) !v
 
     frame.domChanged();
 
+    // Per the "replace all" algorithm, observers get one combined mutation
+    // record with all removed and added nodes, so per-node notification is
+    // suppressed here.
+    const notify = Frame.observers.hasMutationObservers(frame);
+    var removed: std.ArrayList(*Node) = .empty;
+
     // Remove all existing children
     var it = self.childrenIterator();
     while (it.next()) |child| {
-        frame.removeNode(self, child, .{ .will_be_reconnected = false });
+        if (notify) {
+            try removed.append(frame.call_arena, child);
+        }
+        frame.removeNode(self, child, .{ .will_be_reconnected = false, .notify_observers = false });
     }
 
     // Append new children
@@ -1296,7 +1305,11 @@ pub fn replaceChildren(self: *Node, nodes: []const NodeOrText, frame: *Frame) !v
             child_connected = child.isConnected();
             frame.removeNode(previous_parent, child, .{ .will_be_reconnected = parent_is_connected });
         }
-        try frame.appendNode(self, child, .{ .child_already_connected = child_connected });
+        try frame.appendNode(self, child, .{ .child_already_connected = child_connected, .notify_observers = false });
+    }
+
+    if (notify and (removed.items.len > 0 or children_to_add.items.len > 0)) {
+        Frame.observers.notifyChildListChange(frame, self, children_to_add.items, removed.items, null, null);
     }
 }