diff --git a/build.zig.zon b/build.zig.zon
index 9d57095f..036d9ba5 100644
--- a/build.zig.zon
+++ b/build.zig.zon
@@ -5,9 +5,9 @@
     .fingerprint = 0xda130f3af836cea0,
     .dependencies = .{
         .v8 = .{
-            .url = "https://github.com/lightpanda-io/zig-v8-fork/archive/305bb3706716d32d59b2ffa674731556caa1002b.tar.gz",
-            .hash = "v8-0.0.0-xddH63bVAwBSEobaUok9J0er1FqsvEujCDDVy6ItqKQ5",
+            .url = "https://github.com/lightpanda-io/zig-v8-fork/archive/3aa2b39cb1ab588b85970beef5b374effccf1415.tar.gz",
+            .hash = "v8-0.0.0-xddH66TeAwDDEs3QkHFlukxqqrRXITzzmmIn2NHISHCn",
         },
-        //.v8 = .{ .path = "../zig-v8-fork" }
+        // .v8 = .{ .path = "../zig-v8-fork" }
     },
 }
diff --git a/src/browser/Page.zig b/src/browser/Page.zig
index 6555a9d6..ff22f2b6 100644
--- a/src/browser/Page.zig
+++ b/src/browser/Page.zig
@@ -37,6 +37,7 @@ const Scheduler = @import("Scheduler.zig");
 const History = @import("webapi/History.zig");
 const EventManager = @import("EventManager.zig");
 const ScriptManager = @import("ScriptManager.zig");
+
 const polyfill = @import("polyfill/polyfill.zig");
 
 const Parser = @import("parser/Parser.zig");
@@ -50,6 +51,8 @@ const Window = @import("webapi/Window.zig");
 const Location = @import("webapi/Location.zig");
 const Document = @import("webapi/Document.zig");
 const HtmlScript = @import("webapi/Element.zig").Html.Script;
+const MutationObserver = @import("webapi/MutationObserver.zig");
+const IntersectionObserver = @import("webapi/IntersectionObserver.zig");
 const storage = @import("webapi/storage/storage.zig");
 
 const timestamp = @import("../datetime.zig").timestamp;
@@ -89,6 +92,14 @@ _element_class_lists: Element.ClassListLookup = .{},
 
 _script_manager: ScriptManager,
 
+// List of active MutationObservers
+_mutation_observers: std.ArrayList(*MutationObserver) = .{},
+_mutation_delivery_scheduled: bool = false,
+
+// List of active IntersectionObservers
+_intersection_observers: std.ArrayList(*IntersectionObserver) = .{},
+_intersection_delivery_scheduled: bool = false,
+
 _polyfill_loader: polyfill.Loader = .{},
 
 // for heap allocations and managing WebAPI objects
@@ -190,6 +201,11 @@ fn reset(self: *Page, comptime initializing: bool) !void {
     self._notified_network_idle = .init;
     self._notified_network_almost_idle = .init;
 
+    self._mutation_observers = .{};
+    self._mutation_delivery_scheduled = false;
+    self._intersection_observers = .{};
+    self._intersection_delivery_scheduled = false;
+
     try polyfill.preload(self.arena, self.js);
     try self.registerBackgroundTasks();
 }
@@ -677,6 +693,94 @@ pub fn domChanged(self: *Page) void {
     self.version += 1;
 }
 
+pub fn registerMutationObserver(self: *Page, observer: *MutationObserver) !void {
+    try self._mutation_observers.append(self.arena, observer);
+}
+
+pub fn unregisterMutationObserver(self: *Page, observer: *MutationObserver) void {
+    for (self._mutation_observers.items, 0..) |obs, i| {
+        if (obs == observer) {
+            _ = self._mutation_observers.swapRemove(i);
+            return;
+        }
+    }
+}
+
+pub fn registerIntersectionObserver(self: *Page, observer: *IntersectionObserver) !void {
+    try self._intersection_observers.append(self.arena, observer);
+}
+
+pub fn unregisterIntersectionObserver(self: *Page, observer: *IntersectionObserver) void {
+    for (self._intersection_observers.items, 0..) |obs, i| {
+        if (obs == observer) {
+            _ = self._intersection_observers.swapRemove(i);
+            return;
+        }
+    }
+}
+
+pub fn checkIntersections(self: *Page) !void {
+    for (self._intersection_observers.items) |observer| {
+        try observer.checkIntersections(self);
+    }
+}
+
+pub fn scheduleMutationDelivery(self: *Page) !void {
+    // Only queue if not already scheduled
+    if (self._mutation_delivery_scheduled) {
+        return;
+    }
+    self._mutation_delivery_scheduled = true;
+
+    // Queue mutation delivery as a microtask
+    try self.js.queueMutationDelivery();
+}
+
+pub fn scheduleIntersectionDelivery(self: *Page) !void {
+    // Only queue if not already scheduled
+    if (self._intersection_delivery_scheduled) {
+        return;
+    }
+    self._intersection_delivery_scheduled = true;
+
+    // Queue intersection delivery as a microtask
+    try self.js.queueIntersectionDelivery();
+}
+
+pub fn deliverIntersections(self: *Page) void {
+    if (!self._intersection_delivery_scheduled) {
+        return;
+    }
+    self._intersection_delivery_scheduled = false;
+
+    // Iterate backwards to handle observers that disconnect during their callback
+    var i = self._intersection_observers.items.len;
+    while (i > 0) {
+        i -= 1;
+        const observer = self._intersection_observers.items[i];
+        observer.deliverEntries(self) catch |err| {
+            log.err(.page, "page.deliverIntersections", .{ .err = err });
+        };
+    }
+}
+
+pub fn deliverMutations(self: *Page) void {
+    if (!self._mutation_delivery_scheduled) {
+        return;
+    }
+    self._mutation_delivery_scheduled = false;
+
+    // Iterate backwards to handle observers that disconnect during their callback
+    var i = self._mutation_observers.items.len;
+    while (i > 0) {
+        i -= 1;
+        const observer = self._mutation_observers.items[i];
+        observer.deliverRecords(self) catch |err| {
+            log.err(.page, "page.deliverMutations", .{ .err = err });
+        };
+    }
+}
+
 fn notifyNetworkIdle(self: *Page) void {
     std.debug.assert(self._notified_network_idle == .done);
     self._session.browser.notification.dispatch(.page_network_idle, &.{
@@ -1106,6 +1210,10 @@ const RemoveNodeOpts = struct {
     will_be_reconnected: bool,
 };
 pub fn removeNode(self: *Page, parent: *Node, child: *Node, opts: RemoveNodeOpts) void {
+    // Capture siblings before removing
+    const previous_sibling = child.previousSibling();
+    const next_sibling = child.nextSibling();
+
     const children = parent._children.?;
     switch (children.*) {
         .one => |n| {
@@ -1132,6 +1240,11 @@ pub fn removeNode(self: *Page, parent: *Node, child: *Node, opts: RemoveNodeOpts
     child._parent = null;
     child._child_link = .{};
 
+    if (self.hasMutationObservers()) {
+        const removed = [_]*Node{child};
+        self.childListChange(parent, &.{}, &removed, previous_sibling, next_sibling);
+    }
+
     if (opts.will_be_reconnected) {
         // We might be removing the node only to re-insert it. If the node will
         // remain connected, we can skip the expensive process of fully
@@ -1232,10 +1345,30 @@ pub fn _insertNodeRelative(self: *Page, comptime from_parser: bool, parent: *Nod
     }
     child._parent = parent;
 
-    if (comptime from_parser == false) {
-        // When the parser adds the node, nodeIsReady is only called when the
-        // nodeComplete() callback is executed.
-        try self.nodeIsReady(false, child);
+    // Tri-state behavior for mutations:
+    // 1. from_parser=true, parse_mode=document -> no mutations (initial document parse)
+    // 2. from_parser=true, parse_mode=fragment -> mutations (innerHTML additions)
+    // 3. from_parser=false, parse_mode=document -> mutation (js manipulation)
+    // split like this because from_parser can be comptime known.
+    const should_notify = if (comptime from_parser)
+        self._parse_mode == .fragment
+    else
+        true;
+
+    if (should_notify) {
+        if (comptime from_parser == false) {
+            // When the parser adds the node, nodeIsReady is only called when the
+            // nodeComplete() callback is executed.
+            try self.nodeIsReady(false, child);
+        }
+
+        // Notify mutation observers about childList change
+        if (self.hasMutationObservers()) {
+            const previous_sibling = child.previousSibling();
+            const next_sibling = child.nextSibling();
+            const added = [_]*Node{child};
+            self.childListChange(parent, &added, &.{}, previous_sibling, next_sibling);
+        }
     }
 
     var document_by_id = &self.document._elements_by_id;
@@ -1276,16 +1409,71 @@ pub fn _insertNodeRelative(self: *Page, comptime from_parser: bool, parent: *Nod
     }
 }
 
-pub fn attributeChange(self: *Page, element: *Element, name: []const u8, value: []const u8) void {
+pub fn attributeChange(self: *Page, element: *Element, name: []const u8, value: []const u8, old_value: ?[]const u8) void {
     _ = Element.Build.call(element, "attributeChange", .{ element, name, value, self }) catch |err| {
         log.err(.bug, "build.attributeChange", .{ .tag = element.getTag(), .name = name, .value = value, .err = err });
     };
+
+    for (self._mutation_observers.items) |observer| {
+        observer.notifyAttributeChange(element, name, old_value, self) catch |err| {
+            log.err(.page, "attributeChange.notifyObserver", .{ .err = err });
+        };
+    }
 }
 
-pub fn attributeRemove(self: *Page, element: *Element, name: []const u8) void {
+pub fn attributeRemove(self: *Page, element: *Element, name: []const u8, old_value: []const u8) void {
     _ = Element.Build.call(element, "attributeRemove", .{ element, name, self }) catch |err| {
         log.err(.bug, "build.attributeRemove", .{ .tag = element.getTag(), .name = name, .err = err });
     };
+
+    for (self._mutation_observers.items) |observer| {
+        observer.notifyAttributeChange(element, name, old_value, self) catch |err| {
+            log.err(.page, "attributeRemove.notifyObserver", .{ .err = err });
+        };
+    }
+}
+
+pub fn hasMutationObservers(self: *const Page) bool {
+    return self._mutation_observers.items.len > 0;
+}
+
+pub fn characterDataChange(
+    self: *Page,
+    target: *Node,
+    old_value: []const u8,
+) void {
+    // Notify mutation observers
+    for (self._mutation_observers.items) |observer| {
+        observer.notifyCharacterDataChange(target, old_value, self) catch |err| {
+            log.err(.page, "cdataChange.notifyObserver", .{ .err = err });
+        };
+    }
+}
+
+pub fn childListChange(
+    self: *Page,
+    target: *Node,
+    added_nodes: []const *Node,
+    removed_nodes: []const *Node,
+    previous_sibling: ?*Node,
+    next_sibling: ?*Node,
+) void {
+    // Filter out HTML wrapper element during fragment parsing (html5ever quirk)
+    if (self._parse_mode == .fragment and added_nodes.len == 1) {
+        if (added_nodes[0].is(Element.Html.Html) != null) {
+            // This is the temporary HTML wrapper, added by html5ever
+            // that will be unwrapped, see:
+            // https://github.com/servo/html5ever/issues/583
+            return;
+        }
+    }
+
+    // Notify mutation observers
+    for (self._mutation_observers.items) |observer| {
+        observer.notifyChildListChange(target, added_nodes, removed_nodes, previous_sibling, next_sibling, self) catch |err| {
+            log.err(.page, "childListChange.notifyObserver", .{ .err = err });
+        };
+    }
 }
 
 // TODO: optimize and cleanup, this is called a lot (e.g., innerHTML = '')
@@ -1302,9 +1490,22 @@ pub fn parseHtmlAsChildren(self: *Page, node: *Node, html: []const u8) !void {
     const first = children.one;
     std.debug.assert(first.is(Element.Html.Html) != null);
     node._children = first._children;
-    var it = node.childrenIterator();
-    while (it.next()) |child| {
-        child._parent = node;
+
+    if (self.hasMutationObservers()) {
+        var it = node.childrenIterator();
+        while (it.next()) |child| {
+            child._parent = node;
+            // Notify mutation observers for each unwrapped child
+            const previous_sibling = child.previousSibling();
+            const next_sibling = child.nextSibling();
+            const added = [_]*Node{child};
+            self.childListChange(node, &added, &.{}, previous_sibling, next_sibling);
+        }
+    } else {
+        var it = node.childrenIterator();
+        while (it.next()) |child| {
+            child._parent = node;
+        }
     }
 }
 
diff --git a/src/browser/js/Context.zig b/src/browser/js/Context.zig
index 087a4204..4c7ff66b 100644
--- a/src/browser/js/Context.zig
+++ b/src/browser/js/Context.zig
@@ -1907,6 +1907,26 @@ fn zigJsonToJs(isolate: v8.Isolate, v8_context: v8.Context, value: std.json.Valu
     }
 }
 
+// Microtasks
+pub fn queueMutationDelivery(self: *Context) !void {
+    self.isolate.enqueueMicrotask(struct {
+        fn run(data: ?*anyopaque) callconv(.c) void {
+            const page: *Page = @ptrCast(@alignCast(data.?));
+            page.deliverMutations();
+        }
+    }.run, self.page);
+}
+
+pub fn queueIntersectionDelivery(self: *Context) !void {
+    self.isolate.enqueueMicrotask(struct {
+        fn run(data: ?*anyopaque) callconv(.c) void {
+            const page: *Page = @ptrCast(@alignCast(data.?));
+            page.deliverIntersections();
+        }
+    }.run, self.page);
+}
+
+
 // == Misc ==
 // An interface for types that want to have their jsDeinit function to be
 // called when the call context ends
diff --git a/src/browser/js/Inspector.zig b/src/browser/js/Inspector.zig
index d8394428..f04409e8 100644
--- a/src/browser/js/Inspector.zig
+++ b/src/browser/js/Inspector.zig
@@ -46,7 +46,14 @@ pub fn init(allocator: Allocator, isolate: v8.Isolate, ctx: anytype) !Inspector
     // If necessary, turn a void context into something we can safely ptrCast
     const safe_context: *anyopaque = if (ContextT == void) @ptrCast(@constCast(&{})) else ctx;
 
-    const channel = v8.InspectorChannel.init(safe_context, InspectorContainer.onInspectorResponse, InspectorContainer.onInspectorEvent, isolate);
+    const channel = v8.InspectorChannel.init(
+        safe_context,
+        InspectorContainer.onInspectorResponse,
+        InspectorContainer.onInspectorEvent,
+        InspectorContainer.onRunMessageLoopOnPause,
+        InspectorContainer.onQuitMessageLoopOnPause,
+        isolate,
+    );
 
     const client = v8.InspectorClient.init();
 
@@ -127,6 +134,8 @@ pub fn getNodePtr(self: *const Inspector, allocator: Allocator, object_id: []con
 const NoopInspector = struct {
     pub fn onInspectorResponse(_: *anyopaque, _: u32, _: []const u8) void {}
     pub fn onInspectorEvent(_: *anyopaque, _: []const u8) void {}
+    pub fn onRunMessageLoopOnPause(_: *anyopaque, _: u32) void {}
+    pub fn onQuitMessageLoopOnPause(_: *anyopaque) void {}
 };
 
 pub fn getTaggedAnyOpaque(value: v8.Value) ?*js.TaggedAnyOpaque {
diff --git a/src/browser/js/bridge.zig b/src/browser/js/bridge.zig
index 714407eb..447ce3ef 100644
--- a/src/browser/js/bridge.zig
+++ b/src/browser/js/bridge.zig
@@ -544,4 +544,5 @@ pub const JsApis = flattenTypes(&.{
     @import("../webapi/URL.zig"),
     @import("../webapi/Window.zig"),
     @import("../webapi/MutationObserver.zig"),
+    @import("../webapi/IntersectionObserver.zig"),
 });
diff --git a/src/browser/tests/intersection_observer/basic.html b/src/browser/tests/intersection_observer/basic.html
new file mode 100644
index 00000000..4131d1dd
--- /dev/null
+++ b/src/browser/tests/intersection_observer/basic.html
@@ -0,0 +1,31 @@
+<!DOCTYPE html>
+<script src="../testing.js"></script>
+<div id="target" style="width: 100px; height: 100px;">Target Element</div>
+
+<script id="basic">
+    const target = document.getElementById('target');
+    let callbackCalled = false;
+    let entries = null;
+
+    const observer = new IntersectionObserver((observerEntries, obs) => {
+        callbackCalled = true;
+        entries = observerEntries;
+    });
+
+    observer.observe(target);
+
+    testing.eventually(() => {
+        testing.expectEqual(true, callbackCalled);
+        testing.expectEqual(1, entries.length);
+
+        const entry = entries[0];
+
+        testing.expectEqual(target, entry.target);
+        testing.expectEqual('boolean', typeof entry.isIntersecting);
+        testing.expectEqual('number', typeof entry.intersectionRatio);
+        testing.expectEqual('object', typeof entry.boundingClientRect);
+        testing.expectEqual('object', typeof entry.intersectionRect);
+
+        observer.disconnect();
+    });
+</script>
diff --git a/src/browser/tests/intersection_observer/disconnect.html b/src/browser/tests/intersection_observer/disconnect.html
new file mode 100644
index 00000000..6677f906
--- /dev/null
+++ b/src/browser/tests/intersection_observer/disconnect.html
@@ -0,0 +1,30 @@
+<!DOCTYPE html>
+<script src="../testing.js"></script>
+<div id="target" style="width: 100px; height: 100px;">Target Element</div>
+
+<script id="disconnect">
+    const target = document.getElementById('target');
+    let callCount = 0;
+
+    const observer = new IntersectionObserver(() => {
+        callCount++;
+    });
+
+    observer.observe(target);
+
+    testing.eventually(() => {
+        testing.expectEqual(1, callCount);
+
+        observer.disconnect();
+
+        // Create a new observer to trigger another check
+        // If disconnect worked, the old observer won't fire
+        const observer2 = new IntersectionObserver(() => {});
+        observer2.observe(target);
+
+        testing.eventually(() => {
+            observer2.disconnect();
+            testing.expectEqual(1, callCount);
+        });
+    });
+</script>
diff --git a/src/browser/tests/intersection_observer/multiple_targets.html b/src/browser/tests/intersection_observer/multiple_targets.html
new file mode 100644
index 00000000..358a8355
--- /dev/null
+++ b/src/browser/tests/intersection_observer/multiple_targets.html
@@ -0,0 +1,29 @@
+<!DOCTYPE html>
+<script src="../testing.js"></script>
+<div id="target1" style="width: 100px; height: 100px;">Target 1</div>
+<div id="target2" style="width: 100px; height: 100px;">Target 2</div>
+
+<script id="multiple">
+    const target1 = document.getElementById('target1');
+    const target2 = document.getElementById('target2');
+    let entryCount = 0;
+    const seenTargets = new Set();
+
+    const observer = new IntersectionObserver((entries) => {
+        entries.forEach(entry => {
+            entryCount++;
+            seenTargets.add(entry.target);
+        });
+    });
+
+    observer.observe(target1);
+    observer.observe(target2);
+
+    testing.eventually(() => {
+        testing.expectEqual(2, entryCount);
+        testing.expectTrue(seenTargets.has(target1));
+        testing.expectTrue(seenTargets.has(target2));
+
+        observer.disconnect();
+    });
+</script>
diff --git a/src/browser/tests/intersection_observer/unobserve.html b/src/browser/tests/intersection_observer/unobserve.html
new file mode 100644
index 00000000..8d37e2c3
--- /dev/null
+++ b/src/browser/tests/intersection_observer/unobserve.html
@@ -0,0 +1,30 @@
+<!DOCTYPE html>
+<script src="../testing.js"></script>
+<div id="target1" style="width: 100px; height: 100px;">Target 1</div>
+<div id="target2" style="width: 100px; height: 100px;">Target 2</div>
+
+<script id="unobserve">
+    const target1 = document.getElementById('target1');
+    const target2 = document.getElementById('target2');
+    const seenTargets = [];
+
+    const observer = new IntersectionObserver((entries) => {
+        entries.forEach(entry => {
+            seenTargets.push(entry.target);
+        });
+    });
+
+    // Observe target1, unobserve it, then observe target2
+    // We should only get a callback for target2
+    observer.observe(target1);
+    observer.unobserve(target1);
+    observer.observe(target2);
+
+    testing.eventually(() => {
+        // Should only see target2, not target1
+        testing.expectEqual(1, seenTargets.length);
+        testing.expectEqual(target2, seenTargets[0]);
+
+        observer.disconnect();
+    });
+</script>
diff --git a/src/browser/tests/mutation_observer/character_data.html b/src/browser/tests/mutation_observer/character_data.html
new file mode 100644
index 00000000..6afa299c
--- /dev/null
+++ b/src/browser/tests/mutation_observer/character_data.html
@@ -0,0 +1,79 @@
+<!DOCTYPE html>
+<div id="test-element-1">Initial text</div>
+<div id="test-element-2">Test</div>
+<div id="test-element-3">Test</div>
+
+<script src="../testing.js"></script>
+<script id="character_data">
+  testing.async(async () => {
+    const element = document.getElementById('test-element-1');
+    const textNode = element.firstChild;
+
+    let mutations = null;
+    const observer = new MutationObserver((records) => {
+      observer.disconnect();
+      mutations = records;
+    });
+    observer.observe(textNode, { characterData: true });
+
+    textNode.data = 'Changed text';
+
+    Promise.resolve().then(() => {
+      testing.expectEqual(1, mutations.length, {script_id: 'character_data'});
+      testing.expectEqual('characterData', mutations[0].type);
+      testing.expectEqual(textNode, mutations[0].target);
+      testing.expectEqual(null, mutations[0].oldValue);
+    });
+  });
+</script>
+
+<script id="character_data_old_value">
+  testing.async(async () => {
+    const element = document.getElementById('test-element-2');
+    const textNode = element.firstChild;
+
+    let mutations = null;
+    const observer = new MutationObserver((records) => {
+      observer.disconnect();
+      mutations = records;
+    });
+    observer.observe(textNode, { characterData: true, characterDataOldValue: true });
+
+    textNode.data = 'First change';
+    textNode.data = 'Second change';
+
+    Promise.resolve().then(() => {
+      testing.expectEqual(2, mutations.length, {script_id: 'character_data_old_value'});
+
+      testing.expectEqual('characterData', mutations[0].type);
+      testing.expectEqual(textNode, mutations[0].target);
+      testing.expectEqual('Test', mutations[0].oldValue);
+
+      testing.expectEqual('characterData', mutations[1].type);
+      testing.expectEqual(textNode, mutations[1].target);
+      testing.expectEqual('First change', mutations[1].oldValue);
+    });
+  });
+</script>
+
+<script id="character_data_no_observe">
+  testing.async(async () => {
+    const element = document.getElementById('test-element-3');
+    const textNode = element.firstChild;
+
+    let callbackCalled = false;
+    const observer = new MutationObserver(() => {
+      callbackCalled = true;
+    });
+
+    // Observe for attributes, not characterData
+    observer.observe(element, { attributes: true });
+
+    // Change text node data (should not trigger)
+    textNode.data = 'Changed but not observed';
+
+    Promise.resolve().then(() => {
+      testing.expectEqual(false, callbackCalled);
+    });
+  });
+</script>
diff --git a/src/browser/tests/mutation_observer/childlist.html b/src/browser/tests/mutation_observer/childlist.html
new file mode 100644
index 00000000..915dbcf4
--- /dev/null
+++ b/src/browser/tests/mutation_observer/childlist.html
@@ -0,0 +1,327 @@
+<!DOCTYPE html>
+<div id="parent"><div id="child1">Child 1</div></div>
+<div id="empty-parent"></div>
+<div id="remove-parent"><div id="only-child">Only</div></div>
+<div id="text-parent"></div>
+<div id="middle-parent"><div id="first">First</div><div id="middle">Middle</div><div id="last">Last</div></div>
+
+<script src="../testing.js"></script>
+<script id="childlist">
+  testing.async(async () => {
+    const parent = document.getElementById('parent');
+    const child1 = document.getElementById('child1');
+
+    let mutations = null;
+    const observer = new MutationObserver((records) => {
+      observer.disconnect();
+      mutations = records;
+    });
+    observer.observe(parent, { childList: true });
+
+    const child2 = document.createElement('div');
+    child2.textContent = 'Child 2';
+    parent.appendChild(child2);
+
+    Promise.resolve().then(() => {
+      testing.expectEqual(1, mutations.length, {script_id: 'childlist'});
+      testing.expectEqual('childList', mutations[0].type);
+      testing.expectEqual(parent, mutations[0].target);
+      testing.expectEqual(1, mutations[0].addedNodes.length);
+      testing.expectEqual(child2, mutations[0].addedNodes[0]);
+      testing.expectEqual(0, mutations[0].removedNodes.length);
+      testing.expectEqual(child1, mutations[0].previousSibling);
+      testing.expectEqual(null, mutations[0].nextSibling);
+    });
+  });
+</script>
+
+<script id="childlist_empty_parent">
+  testing.async(async () => {
+    const emptyParent = document.getElementById('empty-parent');
+
+    let mutations = null;
+    const observer = new MutationObserver((records) => {
+      observer.disconnect();
+      mutations = records;
+    });
+    observer.observe(emptyParent, { childList: true });
+
+    const firstChild = document.createElement('div');
+    firstChild.textContent = 'First child';
+    emptyParent.appendChild(firstChild);
+
+    Promise.resolve().then(() => {
+      testing.expectEqual(1, mutations.length, {script_id: 'childlist_empty_parent'});
+      testing.expectEqual('childList', mutations[0].type);
+      testing.expectEqual(emptyParent, mutations[0].target);
+      testing.expectEqual(1, mutations[0].addedNodes.length);
+      testing.expectEqual(firstChild, mutations[0].addedNodes[0]);
+      testing.expectEqual(0, mutations[0].removedNodes.length);
+      testing.expectEqual(null, mutations[0].previousSibling);
+      testing.expectEqual(null, mutations[0].nextSibling);
+    });
+  });
+</script>
+
+<script id="childlist_remove_last">
+  testing.async(async () => {
+    const removeParent = document.getElementById('remove-parent');
+    const onlyChild = document.getElementById('only-child');
+
+    let mutations = null;
+    const observer = new MutationObserver((records) => {
+      observer.disconnect();
+      mutations = records;
+    });
+    observer.observe(removeParent, { childList: true });
+
+    removeParent.removeChild(onlyChild);
+
+    Promise.resolve().then(() => {
+      testing.expectEqual(1, mutations.length, {script_id: 'childlist_remove_last'});
+      testing.expectEqual('childList', mutations[0].type);
+      testing.expectEqual(removeParent, mutations[0].target);
+      testing.expectEqual(0, mutations[0].addedNodes.length);
+      testing.expectEqual(1, mutations[0].removedNodes.length);
+      testing.expectEqual(onlyChild, mutations[0].removedNodes[0]);
+      testing.expectEqual(null, mutations[0].previousSibling);
+      testing.expectEqual(null, mutations[0].nextSibling);
+    });
+  });
+</script>
+
+<script id="childlist_text_node">
+  testing.async(async () => {
+    const textParent = document.getElementById('text-parent');
+
+    let mutations = null;
+    const observer = new MutationObserver((records) => {
+      observer.disconnect();
+      mutations = records;
+    });
+    observer.observe(textParent, { childList: true });
+
+    const textNode = document.createTextNode('Hello world');
+    textParent.appendChild(textNode);
+
+    Promise.resolve().then(() => {
+      testing.expectEqual(1, mutations.length, {script_id: 'childlist_text_node'});
+      testing.expectEqual('childList', mutations[0].type);
+      testing.expectEqual(textParent, mutations[0].target);
+      testing.expectEqual(1, mutations[0].addedNodes.length);
+      testing.expectEqual(textNode, mutations[0].addedNodes[0]);
+      testing.expectEqual(null, mutations[0].previousSibling);
+      testing.expectEqual(null, mutations[0].nextSibling);
+    });
+  });
+</script>
+
+<script id="childlist_remove_middle">
+  testing.async(async () => {
+    const middleParent = document.getElementById('middle-parent');
+    const first = document.getElementById('first');
+    const middle = document.getElementById('middle');
+    const last = document.getElementById('last');
+
+    let mutations = null;
+    const observer = new MutationObserver((records) => {
+      observer.disconnect();
+      mutations = records;
+    });
+    observer.observe(middleParent, { childList: true });
+
+    middleParent.removeChild(middle);
+
+    Promise.resolve().then(() => {
+      testing.expectEqual(1, mutations.length, {script_id: 'childlist_remove_middle'});
+      testing.expectEqual('childList', mutations[0].type);
+      testing.expectEqual(middleParent, mutations[0].target);
+      testing.expectEqual(0, mutations[0].addedNodes.length);
+      testing.expectEqual(1, mutations[0].removedNodes.length);
+      testing.expectEqual(middle, mutations[0].removedNodes[0]);
+      testing.expectEqual(first, mutations[0].previousSibling);
+      testing.expectEqual(last, mutations[0].nextSibling);
+    });
+  });
+</script>
+
+<div id="insert-parent"><div id="insert-first">First</div><div id="insert-last">Last</div></div>
+
+<script id="childlist_insert_before">
+  testing.async(async () => {
+    const insertParent = document.getElementById('insert-parent');
+    const insertFirst = document.getElementById('insert-first');
+    const insertLast = document.getElementById('insert-last');
+
+    let mutations = null;
+    const observer = new MutationObserver((records) => {
+      observer.disconnect();
+      mutations = records;
+    });
+    observer.observe(insertParent, { childList: true });
+
+    const insertMiddle = document.createElement('div');
+    insertMiddle.textContent = 'Middle';
+    insertParent.insertBefore(insertMiddle, insertLast);
+
+    Promise.resolve().then(() => {
+      testing.expectEqual(1, mutations.length, {script_id: 'childlist_insert_before'});
+      testing.expectEqual('childList', mutations[0].type);
+      testing.expectEqual(insertParent, mutations[0].target);
+      testing.expectEqual(1, mutations[0].addedNodes.length);
+      testing.expectEqual(insertMiddle, mutations[0].addedNodes[0]);
+      testing.expectEqual(0, mutations[0].removedNodes.length);
+      testing.expectEqual(insertFirst, mutations[0].previousSibling);
+      testing.expectEqual(insertLast, mutations[0].nextSibling);
+    });
+  });
+</script>
+
+<div id="replace-parent"><div id="replace-old">Old</div></div>
+
+<script id="childlist_replace_child">
+  testing.async(async () => {
+    const replaceParent = document.getElementById('replace-parent');
+    const replaceOld = document.getElementById('replace-old');
+
+    let mutations = null;
+    const observer = new MutationObserver((records) => {
+      observer.disconnect();
+      mutations = records;
+    });
+    observer.observe(replaceParent, { childList: true });
+
+    const replaceNew = document.createElement('div');
+    replaceNew.textContent = 'New';
+    replaceParent.replaceChild(replaceNew, replaceOld);
+
+    Promise.resolve().then(() => {
+      // replaceChild generates two separate mutation records in modern spec:
+      // 1. First record for insertBefore (new node added)
+      // 2. Second record for removeChild (old node removed)
+      testing.expectEqual(2, mutations.length, {script_id: 'childlist_replace_child'});
+
+      // First mutation: insertion of new node
+      testing.expectEqual('childList', mutations[0].type);
+      testing.expectEqual(replaceParent, mutations[0].target);
+      testing.expectEqual(1, mutations[0].addedNodes.length);
+      testing.expectEqual(replaceNew, mutations[0].addedNodes[0]);
+      testing.expectEqual(0, mutations[0].removedNodes.length);
+      testing.expectEqual(null, mutations[0].previousSibling);
+      testing.expectEqual(replaceOld, mutations[0].nextSibling);
+
+      // Second mutation: removal of old node
+      testing.expectEqual('childList', mutations[1].type);
+      testing.expectEqual(replaceParent, mutations[1].target);
+      testing.expectEqual(0, mutations[1].addedNodes.length);
+      testing.expectEqual(1, mutations[1].removedNodes.length);
+      testing.expectEqual(replaceOld, mutations[1].removedNodes[0]);
+      testing.expectEqual(replaceNew, mutations[1].previousSibling);
+      testing.expectEqual(null, mutations[1].nextSibling);
+    });
+  });
+</script>
+
+<div id="multiple-parent"></div>
+
+<script id="childlist_multiple_mutations">
+  testing.async(async () => {
+    const multipleParent = document.getElementById('multiple-parent');
+
+    let mutations = null;
+    const observer = new MutationObserver((records) => {
+      observer.disconnect();
+      mutations = records;
+    });
+    observer.observe(multipleParent, { childList: true });
+
+    const child1 = document.createElement('div');
+    child1.textContent = 'Child 1';
+    multipleParent.appendChild(child1);
+
+    const child2 = document.createElement('div');
+    child2.textContent = 'Child 2';
+    multipleParent.appendChild(child2);
+
+    const child3 = document.createElement('div');
+    child3.textContent = 'Child 3';
+    multipleParent.appendChild(child3);
+
+    Promise.resolve().then(() => {
+      testing.expectEqual(3, mutations.length, {script_id: 'childlist_multiple_mutations'});
+
+      testing.expectEqual('childList', mutations[0].type);
+      testing.expectEqual(child1, mutations[0].addedNodes[0]);
+      testing.expectEqual(null, mutations[0].previousSibling);
+      testing.expectEqual(null, mutations[0].nextSibling);
+
+      testing.expectEqual('childList', mutations[1].type);
+      testing.expectEqual(child2, mutations[1].addedNodes[0]);
+      testing.expectEqual(child1, mutations[1].previousSibling);
+      testing.expectEqual(null, mutations[1].nextSibling);
+
+      testing.expectEqual('childList', mutations[2].type);
+      testing.expectEqual(child3, mutations[2].addedNodes[0]);
+      testing.expectEqual(child2, mutations[2].previousSibling);
+      testing.expectEqual(null, mutations[2].nextSibling);
+    });
+  });
+</script>
+
+<div id="inner-html-parent"><div>Old 1</div><div>Old 2</div><div>Old 3</div></div>
+
+<script id="childlist_inner_html">
+  testing.async(async () => {
+    const innerHtmlParent = document.getElementById('inner-html-parent');
+    const oldChildren = Array.from(innerHtmlParent.children);
+
+    let mutations = null;
+    const observer = new MutationObserver((records) => {
+      observer.disconnect();
+      mutations = records;
+    });
+    observer.observe(innerHtmlParent, { childList: true });
+
+    // No whitespace between tags to avoid text node mutations
+    innerHtmlParent.innerHTML = '<span>New 1</span><span>New 2</span><span>New 3</span>';
+
+    Promise.resolve().then(() => {
+      // innerHTML triggers mutations for both removals and additions
+      // With tri-state: from_parser=true + parse_mode=fragment -> mutations fire
+      // HTML wrapper element is filtered out, so: 3 removals + 3 additions = 6
+      testing.expectEqual(6, mutations.length, {script_id: 'childlist_inner_html'});
+
+      // First 3: removals
+      testing.expectEqual('childList', mutations[0].type);
+      testing.expectEqual(1, mutations[0].removedNodes.length);
+      testing.expectEqual(oldChildren[0], mutations[0].removedNodes[0]);
+      testing.expectEqual(0, mutations[0].addedNodes.length);
+
+      testing.expectEqual('childList', mutations[1].type);
+      testing.expectEqual(1, mutations[1].removedNodes.length);
+      testing.expectEqual(oldChildren[1], mutations[1].removedNodes[0]);
+      testing.expectEqual(0, mutations[1].addedNodes.length);
+
+      testing.expectEqual('childList', mutations[2].type);
+      testing.expectEqual(1, mutations[2].removedNodes.length);
+      testing.expectEqual(oldChildren[2], mutations[2].removedNodes[0]);
+      testing.expectEqual(0, mutations[2].addedNodes.length);
+
+      // Last 3: additions (unwrapped span elements)
+      testing.expectEqual('childList', mutations[3].type);
+      testing.expectEqual(0, mutations[3].removedNodes.length);
+      testing.expectEqual(1, mutations[3].addedNodes.length);
+      testing.expectEqual('SPAN', mutations[3].addedNodes[0].nodeName);
+
+      testing.expectEqual('childList', mutations[4].type);
+      testing.expectEqual(0, mutations[4].removedNodes.length);
+      testing.expectEqual(1, mutations[4].addedNodes.length);
+      testing.expectEqual('SPAN', mutations[4].addedNodes[0].nodeName);
+
+      testing.expectEqual('childList', mutations[5].type);
+      testing.expectEqual(0, mutations[5].removedNodes.length);
+      testing.expectEqual(1, mutations[5].addedNodes.length);
+      testing.expectEqual('SPAN', mutations[5].addedNodes[0].nodeName);
+    });
+  });
+</script>
diff --git a/src/browser/tests/mutation_observer/multiple_observers.html b/src/browser/tests/mutation_observer/multiple_observers.html
new file mode 100644
index 00000000..850aee64
--- /dev/null
+++ b/src/browser/tests/mutation_observer/multiple_observers.html
@@ -0,0 +1,47 @@
+<!DOCTYPE html>
+<div id="target">Test</div>
+
+<script src="../testing.js"></script>
+<script id="multiple_observers">
+  testing.async(async () => {
+    const element = document.getElementById('target');
+
+    let records1 = null;
+    let records2 = null;
+    let callbackCount = 0;
+
+    const observer1 = new MutationObserver((records) => {
+      records1 = records;
+      observer1.disconnect();
+      callbackCount++;
+    });
+
+    const observer2 = new MutationObserver((records) => {
+      records2 = records;
+      observer2.disconnect();
+      callbackCount++;
+    });
+
+    observer1.observe(element, { attributes: true });
+    observer2.observe(element, { attributes: true, attributeOldValue: true });
+
+    element.setAttribute('data-foo', 'bar');
+    element.setAttribute('data-foo', 'baz');
+
+    Promise.resolve().then(() => {
+      testing.expectEqual(2, callbackCount);
+      testing.expectEqual(2, records1.length);
+      testing.expectEqual(2, records2.length);
+
+      testing.expectEqual('data-foo', records1[0].attributeName);
+      testing.expectEqual(null, records1[0].oldValue);
+      testing.expectEqual('data-foo', records1[1].attributeName);
+      testing.expectEqual(null, records1[1].oldValue);
+
+      testing.expectEqual('data-foo', records2[0].attributeName);
+      testing.expectEqual(null, records2[0].oldValue);
+      testing.expectEqual('data-foo', records2[1].attributeName);
+      testing.expectEqual('bar', records2[1].oldValue);
+    });
+  });
+</script>
diff --git a/src/browser/tests/mutation_observer/mutation_observer.html b/src/browser/tests/mutation_observer/mutation_observer.html
new file mode 100644
index 00000000..8ced1815
--- /dev/null
+++ b/src/browser/tests/mutation_observer/mutation_observer.html
@@ -0,0 +1,114 @@
+<!DOCTYPE html>
+<div id="test-element-1" class="initial">Test</div>
+<div id="test-element-2">Test</div>
+<div id="test-element-3">Test</div>
+<div id="test-element-4">Test</div>
+
+<script src="../testing.js"></script>
+<script id="mutation_observer">
+  testing.async(async () => {
+    const element = document.getElementById('test-element-1');
+
+    let mutations = null;
+    const observer = new MutationObserver((records) => {
+      observer.disconnect();
+      mutations = records;
+    });
+    observer.observe(element, { attributes: true });
+
+    element.setAttribute('data-foo', 'bar');
+    element.setAttribute('class', 'changed');
+    element.removeAttribute('data-foo');
+
+    Promise.resolve().then(() => {
+      testing.expectEqual(3, mutations.length, {script_id: 'mutation_observer'});
+
+      testing.expectEqual('attributes', mutations[0].type);
+      testing.expectEqual(element, mutations[0].target);
+      testing.expectEqual('data-foo', mutations[0].attributeName);
+      testing.expectEqual(null, mutations[0].oldValue);
+
+      testing.expectEqual('attributes', mutations[1].type);
+      testing.expectEqual(element, mutations[1].target);
+      testing.expectEqual('class', mutations[1].attributeName);
+      testing.expectEqual(null, mutations[1].oldValue);
+
+      testing.expectEqual('attributes', mutations[2].type);
+      testing.expectEqual(element, mutations[2].target);
+      testing.expectEqual('data-foo', mutations[2].attributeName);
+      testing.expectEqual(null, mutations[2].oldValue);
+    });
+  });
+</script>
+
+<script id="mutation_observer_old_value">
+  testing.async(async () => {
+    const element = document.getElementById('test-element-2');
+
+    let mutations = null;
+    const observer = new MutationObserver((records) => {
+      observer.disconnect();
+      mutations = records;
+    });
+    observer.observe(element, { attributes: true, attributeOldValue: true });
+
+    element.setAttribute('data-test', 'value1');
+    element.setAttribute('data-test', 'value2');
+    element.removeAttribute('data-test');
+
+    Promise.resolve().then(() => {
+      testing.expectEqual(3, mutations.length, {script_id: 'mutation_observer_old_value'});
+
+      testing.expectEqual('data-test', mutations[0].attributeName);
+      testing.expectEqual(null, mutations[0].oldValue);
+
+      testing.expectEqual('data-test', mutations[1].attributeName);
+      testing.expectEqual('value1', mutations[1].oldValue);
+
+      testing.expectEqual('data-test', mutations[2].attributeName);
+      testing.expectEqual('value2', mutations[2].oldValue);
+    });
+  });
+</script>
+
+<script id="mutation_observer_disconnect">
+  testing.async(async () => {
+    const element = document.getElementById('test-element-3');
+
+    let callbackCalled = false;
+    const observer = new MutationObserver(() => {
+      callbackCalled = true;
+    });
+
+    observer.observe(element, { attributes: true });
+    observer.disconnect();
+
+    element.setAttribute('data-disconnected', 'test');
+
+    Promise.resolve().then(() => {
+      testing.expectEqual(false, callbackCalled, {script_id: 'mutation_observer_disconnect'});
+    });
+  });
+</script>
+
+<script id="mutation_observer_take_records">
+  testing.async(async () => {
+    const element = document.getElementById('test-element-4');
+
+    let callbackCalled = false;
+    const observer = new MutationObserver(() => {
+      callbackCalled = true;
+    });
+
+    observer.observe(element, { attributes: true });
+    element.setAttribute('data-take', 'records');
+
+    const taken = observer.takeRecords();
+    testing.expectEqual(1, taken.length);
+    testing.expectEqual('data-take', taken[0].attributeName);
+
+    Promise.resolve().then(() => {
+      testing.expectEqual(false, callbackCalled);
+    });
+  });
+</script>
diff --git a/src/browser/tests/mutation_observer/mutations_during_callback.html b/src/browser/tests/mutation_observer/mutations_during_callback.html
new file mode 100644
index 00000000..829f3d5a
--- /dev/null
+++ b/src/browser/tests/mutation_observer/mutations_during_callback.html
@@ -0,0 +1,39 @@
+<!DOCTYPE html>
+<div id="target">Test</div>
+
+<script src="../testing.js"></script>
+<script id="mutations_during_callback">
+  testing.async(async () => {
+    const element = document.getElementById('target');
+
+    let callCount = 0;
+    let firstRecords = null;
+    let secondRecords = null;
+
+    const observer = new MutationObserver((records) => {
+      callCount++;
+      if (callCount === 1) {
+        firstRecords = records;
+        element.setAttribute('data-second', 'from-callback');
+      } else if (callCount === 2) {
+        secondRecords = records;
+        observer.disconnect();
+      }
+    });
+
+    observer.observe(element, { attributes: true });
+
+    element.setAttribute('data-first', 'initial');
+
+    Promise.resolve().then(() => {
+      // After first microtask, first callback should have run and triggered second mutation
+    }).then(() => {
+      // After second microtask, second callback should have run
+      testing.expectEqual(2, callCount, {script_id: 'mutations_during_callback'});
+      testing.expectEqual(1, firstRecords.length);
+      testing.expectEqual('data-first', firstRecords[0].attributeName);
+      testing.expectEqual(1, secondRecords.length);
+      testing.expectEqual('data-second', secondRecords[0].attributeName);
+    });
+  });
+</script>
diff --git a/src/browser/tests/mutation_observer/observe_multiple_targets.html b/src/browser/tests/mutation_observer/observe_multiple_targets.html
new file mode 100644
index 00000000..c4547526
--- /dev/null
+++ b/src/browser/tests/mutation_observer/observe_multiple_targets.html
@@ -0,0 +1,31 @@
+<!DOCTYPE html>
+<div id="target1">Test1</div>
+<div id="target2">Test2</div>
+
+<script src="../testing.js"></script>
+<script id="observe_multiple_targets">
+  testing.async(async () => {
+    const element1 = document.getElementById('target1');
+    const element2 = document.getElementById('target2');
+
+    let mutations = null;
+    const observer = new MutationObserver((records) => {
+      observer.disconnect();
+      mutations = records;
+    });
+
+    observer.observe(element1, { attributes: true });
+    observer.observe(element2, { attributes: true });
+
+    element1.setAttribute('data-one', 'value1');
+    element2.setAttribute('data-two', 'value2');
+
+    Promise.resolve().then(() => {
+      testing.expectEqual(2, mutations.length);
+      testing.expectEqual(element1, mutations[0].target);
+      testing.expectEqual('data-one', mutations[0].attributeName);
+      testing.expectEqual(element2, mutations[1].target);
+      testing.expectEqual('data-two', mutations[1].attributeName);
+    });
+  });
+</script>
diff --git a/src/browser/tests/mutation_observer/reobserve_same_target.html b/src/browser/tests/mutation_observer/reobserve_same_target.html
new file mode 100644
index 00000000..9249fbac
--- /dev/null
+++ b/src/browser/tests/mutation_observer/reobserve_same_target.html
@@ -0,0 +1,27 @@
+<!DOCTYPE html>
+<div id="target">Test</div>
+
+<script src="../testing.js"></script>
+<script id="reobserve_same_target">
+  testing.async(async () => {
+    const element = document.getElementById('target');
+
+    let mutations = null;
+    const observer = new MutationObserver((records) => {
+      observer.disconnect();
+      mutations = records;
+    });
+
+    observer.observe(element, { attributes: true, attributeOldValue: false });
+    observer.observe(element, { attributes: true, attributeOldValue: true });
+
+    element.setAttribute('data-foo', 'value1');
+    element.setAttribute('data-foo', 'value2');
+
+    Promise.resolve().then(() => {
+      testing.expectEqual(2, mutations.length);
+      testing.expectEqual(null, mutations[0].oldValue);
+      testing.expectEqual('value1', mutations[1].oldValue);
+    });
+  });
+</script>
diff --git a/src/browser/webapi/CData.zig b/src/browser/webapi/CData.zig
index a0b569e8..cb39a70b 100644
--- a/src/browser/webapi/CData.zig
+++ b/src/browser/webapi/CData.zig
@@ -61,11 +61,15 @@ pub fn getData(self: *const CData) []const u8 {
 }
 
 pub fn setData(self: *CData, value: ?[]const u8, page: *Page) !void {
+    const old_value = self._data;
+
     if (value) |v| {
         self._data = try page.dupeString(v);
     } else {
         self._data = "";
     }
+
+    page.characterDataChange(self.asNode(), old_value);
 }
 
 pub fn format(self: *const CData, writer: *std.io.Writer) !void {
diff --git a/src/browser/webapi/IntersectionObserver.zig b/src/browser/webapi/IntersectionObserver.zig
new file mode 100644
index 00000000..7198dfa7
--- /dev/null
+++ b/src/browser/webapi/IntersectionObserver.zig
@@ -0,0 +1,342 @@
+// Copyright (C) 2023-2025  Lightpanda (Selecy SAS)
+//
+// Francis Bouvier <francis@lightpanda.io>
+// Pierre Tachoire <pierre@lightpanda.io>
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as
+// published by the Free Software Foundation, either version 3 of the
+// License, or (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+const std = @import("std");
+const js = @import("../js/js.zig");
+const Page = @import("../Page.zig");
+const Element = @import("Element.zig");
+const DOMRect = @import("DOMRect.zig");
+
+pub fn registerTypes() []const type {
+    return &.{
+        IntersectionObserver,
+        IntersectionObserverEntry,
+    };
+}
+
+const IntersectionObserver = @This();
+
+_callback: js.Function,
+_observing: std.ArrayList(*Element) = .{},
+_root: ?*Element = null,
+_root_margin: []const u8 = "0px",
+_threshold: []const f64 = &.{0.0},
+_pending_entries: std.ArrayList(*IntersectionObserverEntry) = .{},
+_previous_states: std.AutoHashMapUnmanaged(*Element, bool) = .{},
+
+// Shared zero DOMRect to avoid repeated allocations for non-intersecting elements
+var zero_rect: DOMRect = .{
+    ._x = 0.0,
+    ._y = 0.0,
+    ._width = 0.0,
+    ._height = 0.0,
+    ._top = 0.0,
+    ._right = 0.0,
+    ._bottom = 0.0,
+    ._left = 0.0,
+};
+
+pub const ObserverInit = struct {
+    root: ?*Element = null,
+    rootMargin: ?[]const u8 = null,
+    threshold: []const f64 = &.{0.0},
+};
+
+pub fn init(callback: js.Function, options: ?ObserverInit, page: *Page) !*IntersectionObserver {
+    const opts = options orelse ObserverInit{};
+    const root_margin = if (opts.rootMargin) |rm| try page.arena.dupe(u8, rm) else "0px";
+
+    return page._factory.create(IntersectionObserver{
+        ._callback = callback,
+        ._root = opts.root,
+        ._root_margin = root_margin,
+        ._threshold = try page.arena.dupe(f64, opts.threshold),
+    });
+}
+
+pub fn observe(self: *IntersectionObserver, target: *Element, page: *Page) !void {
+    // Check if already observing this target
+    for (self._observing.items) |elem| {
+        if (elem == target) {
+            return;
+        }
+    }
+
+    // Register with page if this is our first observation
+    if (self._observing.items.len == 0) {
+        try page.registerIntersectionObserver(self);
+    }
+
+    try self._observing.append(page.arena, target);
+
+    // Don't initialize previous state yet - let checkIntersection do it
+    // This ensures we get an entry on first observation
+
+    // Check intersection for this new target and schedule delivery
+    try self.checkIntersection(target, page);
+    if (self._pending_entries.items.len > 0) {
+        try page.scheduleIntersectionDelivery();
+    }
+}
+
+pub fn unobserve(self: *IntersectionObserver, target: *Element) void {
+    for (self._observing.items, 0..) |elem, i| {
+        if (elem == target) {
+            _ = self._observing.swapRemove(i);
+            _ = self._previous_states.remove(target);
+
+            // Remove any pending entries for this target
+            var j: usize = 0;
+            while (j < self._pending_entries.items.len) {
+                if (self._pending_entries.items[j]._target == target) {
+                    _ = self._pending_entries.swapRemove(j);
+                } else {
+                    j += 1;
+                }
+            }
+            return;
+        }
+    }
+}
+
+pub fn disconnect(self: *IntersectionObserver, page: *Page) void {
+    page.unregisterIntersectionObserver(self);
+    self._observing.clearRetainingCapacity();
+    self._previous_states.clearRetainingCapacity();
+    self._pending_entries.clearRetainingCapacity();
+}
+
+pub fn takeRecords(self: *IntersectionObserver, page: *Page) ![]*IntersectionObserverEntry {
+    const entries = try page.call_arena.dupe(*IntersectionObserverEntry, self._pending_entries.items);
+    self._pending_entries.clearRetainingCapacity();
+    return entries;
+}
+
+fn calculateIntersection(
+    self: *IntersectionObserver,
+    target: *Element,
+    page: *Page,
+) !IntersectionData {
+    const target_rect = try target.getBoundingClientRect(page);
+
+    // Use root element's rect or viewport (simplified: assume infinite viewport)
+    const root_rect = if (self._root) |root|
+        try root.getBoundingClientRect(page)
+    else
+        // Simplified viewport - assume 1920x1080 for now
+        try page._factory.create(DOMRect{
+            ._x = 0.0,
+            ._y = 0.0,
+            ._width = 1920.0,
+            ._height = 1080.0,
+            ._top = 0.0,
+            ._right = 1920.0,
+            ._bottom = 1080.0,
+            ._left = 0.0,
+        });
+
+    // Calculate intersection rectangle
+    const left = @max(target_rect._left, root_rect._left);
+    const top = @max(target_rect._top, root_rect._top);
+    const right = @min(target_rect._right, root_rect._right);
+    const bottom = @min(target_rect._bottom, root_rect._bottom);
+
+    const is_intersecting = left < right and top < bottom;
+
+    var intersection_rect: ?*DOMRect = null;
+    var intersection_ratio: f64 = 0.0;
+
+    if (is_intersecting) {
+        const width = right - left;
+        const height = bottom - top;
+        const intersection_area = width * height;
+        const target_area = target_rect._width * target_rect._height;
+
+        if (target_area > 0) {
+            intersection_ratio = intersection_area / target_area;
+        }
+
+        intersection_rect = try page._factory.create(DOMRect{
+            ._x = left,
+            ._y = top,
+            ._width = width,
+            ._height = height,
+            ._top = top,
+            ._right = right,
+            ._bottom = bottom,
+            ._left = left,
+        });
+    } else {
+        // No intersection - reuse shared zero rect to avoid allocation
+        intersection_rect = &zero_rect;
+    }
+
+    return .{
+        .is_intersecting = is_intersecting,
+        .intersection_ratio = intersection_ratio,
+        .intersection_rect = intersection_rect.?,
+        .bounding_client_rect = target_rect,
+        .root_bounds = root_rect,
+    };
+}
+
+const IntersectionData = struct {
+    is_intersecting: bool,
+    intersection_ratio: f64,
+    intersection_rect: *DOMRect,
+    bounding_client_rect: *DOMRect,
+    root_bounds: *DOMRect,
+};
+
+fn meetsThreshold(self: *IntersectionObserver, ratio: f64) bool {
+    for (self._threshold) |threshold| {
+        if (ratio >= threshold) {
+            return true;
+        }
+    }
+    return false;
+}
+
+fn checkIntersection(self: *IntersectionObserver, target: *Element, page: *Page) !void {
+    const data = try self.calculateIntersection(target, page);
+    const was_intersecting_opt = self._previous_states.get(target);
+    const is_now_intersecting = data.is_intersecting and self.meetsThreshold(data.intersection_ratio);
+
+    // Create entry if:
+    // 1. First time observing this target (was_intersecting_opt == null)
+    // 2. State changed
+    // 3. Currently intersecting
+    const should_report = was_intersecting_opt == null or
+        was_intersecting_opt.? != is_now_intersecting;
+
+    if (should_report) {
+        const entry = try page.arena.create(IntersectionObserverEntry);
+        entry.* = .{
+            ._target = target,
+            ._time = 0.0, // TODO: Get actual timestamp
+            ._bounding_client_rect = data.bounding_client_rect,
+            ._intersection_rect = data.intersection_rect,
+            ._root_bounds = data.root_bounds,
+            ._intersection_ratio = data.intersection_ratio,
+            ._is_intersecting = is_now_intersecting,
+        };
+
+        try self._pending_entries.append(page.arena, entry);
+        try self._previous_states.put(page.arena, target, is_now_intersecting);
+    }
+}
+
+pub fn checkIntersections(self: *IntersectionObserver, page: *Page) !void {
+    if (self._observing.items.len == 0) {
+        return;
+    }
+
+    for (self._observing.items) |target| {
+        try self.checkIntersection(target, page);
+    }
+
+    if (self._pending_entries.items.len > 0) {
+        try page.scheduleIntersectionDelivery();
+    }
+}
+
+pub fn deliverEntries(self: *IntersectionObserver, page: *Page) !void {
+    if (self._pending_entries.items.len == 0) {
+        return;
+    }
+
+    const entries = try self.takeRecords(page);
+    try self._callback.call(void, .{ entries, self });
+}
+
+pub const IntersectionObserverEntry = struct {
+    _target: *Element,
+    _time: f64,
+    _bounding_client_rect: *DOMRect,
+    _intersection_rect: *DOMRect,
+    _root_bounds: *DOMRect,
+    _intersection_ratio: f64,
+    _is_intersecting: bool,
+
+    pub fn getTarget(self: *const IntersectionObserverEntry) *Element {
+        return self._target;
+    }
+
+    pub fn getTime(self: *const IntersectionObserverEntry) f64 {
+        return self._time;
+    }
+
+    pub fn getBoundingClientRect(self: *const IntersectionObserverEntry) *DOMRect {
+        return self._bounding_client_rect;
+    }
+
+    pub fn getIntersectionRect(self: *const IntersectionObserverEntry) *DOMRect {
+        return self._intersection_rect;
+    }
+
+    pub fn getRootBounds(self: *const IntersectionObserverEntry) ?*DOMRect {
+        return self._root_bounds;
+    }
+
+    pub fn getIntersectionRatio(self: *const IntersectionObserverEntry) f64 {
+        return self._intersection_ratio;
+    }
+
+    pub fn getIsIntersecting(self: *const IntersectionObserverEntry) bool {
+        return self._is_intersecting;
+    }
+
+    pub const JsApi = struct {
+        pub const bridge = js.Bridge(IntersectionObserverEntry);
+
+        pub const Meta = struct {
+            pub const name = "IntersectionObserverEntry";
+            pub const prototype_chain = bridge.prototypeChain();
+            pub var class_id: bridge.ClassId = undefined;
+        };
+
+        pub const target = bridge.accessor(IntersectionObserverEntry.getTarget, null, .{});
+        pub const time = bridge.accessor(IntersectionObserverEntry.getTime, null, .{});
+        pub const boundingClientRect = bridge.accessor(IntersectionObserverEntry.getBoundingClientRect, null, .{});
+        pub const intersectionRect = bridge.accessor(IntersectionObserverEntry.getIntersectionRect, null, .{});
+        pub const rootBounds = bridge.accessor(IntersectionObserverEntry.getRootBounds, null, .{});
+        pub const intersectionRatio = bridge.accessor(IntersectionObserverEntry.getIntersectionRatio, null, .{});
+        pub const isIntersecting = bridge.accessor(IntersectionObserverEntry.getIsIntersecting, null, .{});
+    };
+};
+
+pub const JsApi = struct {
+    pub const bridge = js.Bridge(IntersectionObserver);
+
+    pub const Meta = struct {
+        pub const name = "IntersectionObserver";
+        pub const prototype_chain = bridge.prototypeChain();
+        pub var class_id: bridge.ClassId = undefined;
+    };
+
+    pub const constructor = bridge.constructor(init, .{});
+
+    pub const observe = bridge.function(IntersectionObserver.observe, .{});
+    pub const unobserve = bridge.function(IntersectionObserver.unobserve, .{});
+    pub const disconnect = bridge.function(IntersectionObserver.disconnect, .{});
+    pub const takeRecords = bridge.function(IntersectionObserver.takeRecords, .{});
+};
+
+const testing = @import("../../testing.zig");
+test "WebApi: IntersectionObserver" {
+    try testing.htmlRunner("intersection_observer", .{});
+}
diff --git a/src/browser/webapi/MutationObserver.zig b/src/browser/webapi/MutationObserver.zig
index e33f3223..ed34d80f 100644
--- a/src/browser/webapi/MutationObserver.zig
+++ b/src/browser/webapi/MutationObserver.zig
@@ -16,18 +16,271 @@
 // You should have received a copy of the GNU Affero General Public License
 // along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
+const std = @import("std");
 const js = @import("../js/js.zig");
+const Page = @import("../Page.zig");
+const Node = @import("Node.zig");
+const Element = @import("Element.zig");
+
+pub fn registerTypes() []const type {
+    return &.{
+        MutationObserver,
+        MutationRecord,
+    };
+}
 
-// @ZIGDOM (haha, bet you wish you hadn't opened this file)
-// puppeteer's startup script creates a MutationObserver, even if it doesn't use
-// it in simple scripts. This not-even-a-skeleton is required for puppeteer/cdp.js
-// to run
 const MutationObserver = @This();
 
-pub fn init() MutationObserver {
-    return .{};
+_callback: js.Function,
+_observing: std.ArrayList(Observing) = .{},
+_pending_records: std.ArrayList(*MutationRecord) = .{},
+
+const Observing = struct {
+    target: *Node,
+    options: ObserveOptions,
+};
+
+pub const ObserveOptions = struct {
+    attributes: bool = false,
+    attributeOldValue: bool = false,
+    childList: bool = false,
+    characterData: bool = false,
+    characterDataOldValue: bool = false,
+    // Future: subtree, attributeFilter
+};
+
+pub fn init(callback: js.Function, page: *Page) !*MutationObserver {
+    return page._factory.create(MutationObserver{
+        ._callback = callback,
+    });
 }
 
+pub fn observe(self: *MutationObserver, target: *Node, options: ObserveOptions, page: *Page) !void {
+    // Check if already observing this target
+    for (self._observing.items) |*obs| {
+        if (obs.target == target) {
+            obs.options = options;
+            return;
+        }
+    }
+
+    // Register with page if this is our first observation
+    if (self._observing.items.len == 0) {
+        try page.registerMutationObserver(self);
+    }
+
+    try self._observing.append(page.arena, .{
+        .target = target,
+        .options = options,
+    });
+}
+
+pub fn disconnect(self: *MutationObserver, page: *Page) void {
+    page.unregisterMutationObserver(self);
+    self._observing.clearRetainingCapacity();
+    self._pending_records.clearRetainingCapacity();
+}
+
+pub fn takeRecords(self: *MutationObserver, page: *Page) ![]*MutationRecord {
+    const records = try page.call_arena.dupe(*MutationRecord, self._pending_records.items);
+    self._pending_records.clearRetainingCapacity();
+    return records;
+}
+
+// Called when an attribute changes on any element
+pub fn notifyAttributeChange(
+    self: *MutationObserver,
+    target: *Element,
+    attribute_name: []const u8,
+    old_value: ?[]const u8,
+    page: *Page,
+) !void {
+    const target_node = target.asNode();
+
+    for (self._observing.items) |obs| {
+        if (obs.target != target_node) {
+            continue;
+        }
+        if (!obs.options.attributes) {
+            continue;
+        }
+
+        const record = try page._factory.create(MutationRecord{
+            ._type = .attributes,
+            ._target = target_node,
+            ._attribute_name = try page.arena.dupe(u8, attribute_name),
+            ._old_value = if (obs.options.attributeOldValue and old_value != null)
+                try page.arena.dupe(u8, old_value.?)
+            else
+                null,
+            ._added_nodes = &.{},
+            ._removed_nodes = &.{},
+            ._previous_sibling = null,
+            ._next_sibling = null,
+        });
+
+        try self._pending_records.append(page.arena, record);
+
+        try page.scheduleMutationDelivery();
+        break;
+    }
+}
+
+// Called when character data changes on a text node
+pub fn notifyCharacterDataChange(
+    self: *MutationObserver,
+    target: *Node,
+    old_value: ?[]const u8,
+    page: *Page,
+) !void {
+    for (self._observing.items) |obs| {
+        if (obs.target != target) {
+            continue;
+        }
+        if (!obs.options.characterData) {
+            continue;
+        }
+
+        const record = try page._factory.create(MutationRecord{
+            ._type = .characterData,
+            ._target = target,
+            ._attribute_name = null,
+            ._old_value = if (obs.options.characterDataOldValue and old_value != null)
+                try page.arena.dupe(u8, old_value.?)
+            else
+                null,
+            ._added_nodes = &.{},
+            ._removed_nodes = &.{},
+            ._previous_sibling = null,
+            ._next_sibling = null,
+        });
+
+        try self._pending_records.append(page.arena, record);
+
+        try page.scheduleMutationDelivery();
+        break;
+    }
+}
+
+// Called when children are added or removed from a node
+pub fn notifyChildListChange(
+    self: *MutationObserver,
+    target: *Node,
+    added_nodes: []const *Node,
+    removed_nodes: []const *Node,
+    previous_sibling: ?*Node,
+    next_sibling: ?*Node,
+    page: *Page,
+) !void {
+    for (self._observing.items) |obs| {
+        if (obs.target != target) {
+            continue;
+        }
+        if (!obs.options.childList) {
+            continue;
+        }
+
+        const record = try page._factory.create(MutationRecord{
+            ._type = .childList,
+            ._target = target,
+            ._attribute_name = null,
+            ._old_value = null,
+            ._added_nodes = try page.arena.dupe(*Node, added_nodes),
+            ._removed_nodes = try page.arena.dupe(*Node, removed_nodes),
+            ._previous_sibling = previous_sibling,
+            ._next_sibling = next_sibling,
+        });
+
+        try self._pending_records.append(page.arena, record);
+
+        try page.scheduleMutationDelivery();
+        break;
+    }
+}
+
+pub fn deliverRecords(self: *MutationObserver, page: *Page) !void {
+    if (self._pending_records.items.len == 0) {
+        return;
+    }
+
+    // Take a copy of the records and clear the list before calling callback
+    // This ensures mutations triggered during the callback go into a fresh list
+    const records = try self.takeRecords(page);
+    try self._callback.call(void, .{ records, self });
+}
+
+pub const MutationRecord = struct {
+    _type: Type,
+    _target: *Node,
+    _attribute_name: ?[]const u8,
+    _old_value: ?[]const u8,
+    _added_nodes: []const *Node,
+    _removed_nodes: []const *Node,
+    _previous_sibling: ?*Node,
+    _next_sibling: ?*Node,
+
+    pub const Type = enum {
+        attributes,
+        childList,
+        characterData,
+    };
+
+    pub fn getType(self: *const MutationRecord) []const u8 {
+        return switch (self._type) {
+            .attributes => "attributes",
+            .childList => "childList",
+            .characterData => "characterData",
+        };
+    }
+
+    pub fn getTarget(self: *const MutationRecord) *Node {
+        return self._target;
+    }
+
+    pub fn getAttributeName(self: *const MutationRecord) ?[]const u8 {
+        return self._attribute_name;
+    }
+
+    pub fn getOldValue(self: *const MutationRecord) ?[]const u8 {
+        return self._old_value;
+    }
+
+    pub fn getAddedNodes(self: *const MutationRecord) []const *Node {
+        return self._added_nodes;
+    }
+
+    pub fn getRemovedNodes(self: *const MutationRecord) []const *Node {
+        return self._removed_nodes;
+    }
+
+    pub fn getPreviousSibling(self: *const MutationRecord) ?*Node {
+        return self._previous_sibling;
+    }
+
+    pub fn getNextSibling(self: *const MutationRecord) ?*Node {
+        return self._next_sibling;
+    }
+
+    pub const JsApi = struct {
+        pub const bridge = js.Bridge(MutationRecord);
+
+        pub const Meta = struct {
+            pub const name = "MutationRecord";
+            pub const prototype_chain = bridge.prototypeChain();
+            pub var class_id: bridge.ClassId = undefined;
+        };
+
+        pub const @"type" = bridge.accessor(MutationRecord.getType, null, .{});
+        pub const target = bridge.accessor(MutationRecord.getTarget, null, .{});
+        pub const attributeName = bridge.accessor(MutationRecord.getAttributeName, null, .{});
+        pub const oldValue = bridge.accessor(MutationRecord.getOldValue, null, .{});
+        pub const addedNodes = bridge.accessor(MutationRecord.getAddedNodes, null, .{});
+        pub const removedNodes = bridge.accessor(MutationRecord.getRemovedNodes, null, .{});
+        pub const previousSibling = bridge.accessor(MutationRecord.getPreviousSibling, null, .{});
+        pub const nextSibling = bridge.accessor(MutationRecord.getNextSibling, null, .{});
+    };
+};
+
 pub const JsApi = struct {
     pub const bridge = js.Bridge(MutationObserver);
 
@@ -38,4 +291,13 @@ pub const JsApi = struct {
     };
 
     pub const constructor = bridge.constructor(MutationObserver.init, .{});
+
+    pub const observe = bridge.function(MutationObserver.observe, .{});
+    pub const disconnect = bridge.function(MutationObserver.disconnect, .{});
+    pub const takeRecords = bridge.function(MutationObserver.takeRecords, .{});
 };
+
+const testing = @import("../../testing.zig");
+test "WebApi: MutationObserver" {
+    try testing.htmlRunner("mutation_observer", .{});
+}
diff --git a/src/browser/webapi/element/Attribute.zig b/src/browser/webapi/element/Attribute.zig
index 3d37f173..c07c68dd 100644
--- a/src/browser/webapi/element/Attribute.zig
+++ b/src/browser/webapi/element/Attribute.zig
@@ -156,7 +156,9 @@ pub const List = struct {
         const is_id = isIdForConnected(result.normalized, element);
 
         var entry: *Entry = undefined;
+        var old_value: ?[]const u8 = null;
         if (result.entry) |e| {
+            old_value = try page.call_arena.dupe(u8, e._value.str());
             if (is_id) {
                 _ = page.document._elements_by_id.remove(e._value.str());
             }
@@ -174,7 +176,7 @@ pub const List = struct {
         if (is_id) {
             try page.document._elements_by_id.put(page.arena, entry._value.str(), element);
         }
-        page.attributeChange(element, result.normalized, entry._value.str());
+        page.attributeChange(element, result.normalized, entry._value.str(), old_value);
         return entry;
     }
 
@@ -226,12 +228,13 @@ pub const List = struct {
         const entry = result.entry orelse return;
 
         const is_id = isIdForConnected(result.normalized, element);
+        const old_value = entry._value.str();
 
         if (is_id) {
             _ = page.document._elements_by_id.remove(entry._value.str());
         }
 
-        page.attributeRemove(element, result.normalized);
+        page.attributeRemove(element, result.normalized, old_value);
         _ = page._attribute_lookup.remove(@intFromPtr(entry));
         self._list.remove(&entry._node);
         page._factory.destroy(entry);
diff --git a/src/cdp/cdp.zig b/src/cdp/cdp.zig
index 73c5e514..dc897346 100644
--- a/src/cdp/cdp.zig
+++ b/src/cdp/cdp.zig
@@ -642,6 +642,19 @@ pub fn BrowserContext(comptime CDP_T: type) type {
             };
         }
 
+        // debugger events
+
+
+        pub fn onRunMessageLoopOnPause(_: *anyopaque, _: u32) void {
+            // onRunMessageLoopOnPause is called when a breakpoint is hit.
+            // Until quit pause, we must continue to run a nested message loop
+            // to interact with the the debugger ony (ie. Chrome DevTools).
+        }
+
+        pub fn onQuitMessageLoopOnPause(_: *anyopaque) void {
+            // Quit breakpoint pause.
+        }
+
         // This is hacky x 2. First, we create the JSON payload by gluing our
         // session_id onto it. Second, we're much more client/websocket aware than
         // we should be.