diff --git a/src/browser/Factory.zig b/src/browser/Factory.zig
index d4f7428b..89f76f42 100644
--- a/src/browser/Factory.zig
+++ b/src/browser/Factory.zig
@@ -121,6 +121,16 @@ pub fn document(self: *Factory, child: anytype) !*@TypeOf(child) {
     return child_ptr;
 }
 
+pub fn documentFragment(self: *Factory, child: anytype) !*@TypeOf(child) {
+    const child_ptr = try self.createT(@TypeOf(child));
+    child_ptr.* = child;
+    child_ptr._proto = try self.node(Node.DocumentFragment{
+        ._proto = undefined,
+        ._type = unionInit(Node.DocumentFragment.Type, child_ptr),
+    });
+    return child_ptr;
+}
+
 pub fn element(self: *Factory, child: anytype) !*@TypeOf(child) {
     const child_ptr = try self.createT(@TypeOf(child));
     child_ptr.* = child;
diff --git a/src/browser/Page.zig b/src/browser/Page.zig
index 33e1e565..079e4006 100644
--- a/src/browser/Page.zig
+++ b/src/browser/Page.zig
@@ -50,6 +50,8 @@ const Element = @import("webapi/Element.zig");
 const Window = @import("webapi/Window.zig");
 const Location = @import("webapi/Location.zig");
 const Document = @import("webapi/Document.zig");
+const DocumentFragment = @import("webapi/DocumentFragment.zig");
+const ShadowRoot = @import("webapi/ShadowRoot.zig");
 const Performance = @import("webapi/Performance.zig");
 const HtmlScript = @import("webapi/Element.zig").Html.Script;
 const MutationObserver = @import("webapi/MutationObserver.zig");
@@ -91,6 +93,7 @@ _attribute_named_node_map_lookup: std.AutoHashMapUnmanaged(usize, *Element.Attri
 _element_styles: Element.StyleLookup = .{},
 _element_datasets: Element.DatasetLookup = .{},
 _element_class_lists: Element.ClassListLookup = .{},
+_element_shadow_roots: Element.ShadowRootLookup = .{},
 
 _script_manager: ScriptManager,
 
@@ -211,6 +214,7 @@ fn reset(self: *Page, comptime initializing: bool) !void {
     self._element_styles = .{};
     self._element_datasets = .{};
     self._element_class_lists = .{};
+    self._element_shadow_roots = .{};
     self._notified_network_idle = .init;
     self._notified_network_almost_idle = .init;
 
@@ -714,6 +718,39 @@ pub fn domChanged(self: *Page) void {
     self.version += 1;
 }
 
+pub fn getElementIdMap(page: *Page, node: *Node) *std.StringHashMapUnmanaged(*Element) {
+    if (node.is(ShadowRoot)) |shadow_root| {
+        return &shadow_root._elements_by_id;
+    }
+
+    var parent = node._parent;
+    while (parent) |n| {
+        if (n.is(ShadowRoot)) |shadow_root| {
+            return &shadow_root._elements_by_id;
+        }
+        parent = n._parent;
+    }
+    return &page.document._elements_by_id;
+}
+
+pub fn addElementId(self: *Page, parent: *Node, element: *Element, id: []const u8) !void {
+    var id_map = self.getElementIdMap(parent);
+    const gop = try id_map.getOrPut(self.arena, id);
+    if (!gop.found_existing) {
+        gop.value_ptr.* = element;
+    }
+}
+
+pub fn removeElementId(self: *Page, element: *Element, id: []const u8) void {
+    var id_map = self.getElementIdMap(element.asNode());
+    _ = id_map.remove(id);
+}
+
+pub fn getElementByIdFromNode(self: *Page, node: *Node, id: []const u8) ?*Element {
+    const id_map = self.getElementIdMap(node);
+    return id_map.get(id);
+}
+
 pub fn registerMutationObserver(self: *Page, observer: *MutationObserver) !void {
     try self._mutation_observers.append(self.arena, observer);
 }
@@ -1314,12 +1351,11 @@ pub fn removeNode(self: *Page, parent: *Node, child: *Node, opts: RemoveNodeOpts
 
     // The child was connected and now it no longer is. We need to "disconnect"
     // it and all of its descendants. For now "disconnect" just means updating
-    // document._elements_by_id and invoking disconnectedCallback for custom elements
-    var elements_by_id = &self.document._elements_by_id;
+    // the ID map and invoking disconnectedCallback for custom elements
     var tw = @import("webapi/TreeWalker.zig").Full.Elements.init(child, .{});
     while (tw.next()) |el| {
         if (el.getAttributeSafe("id")) |id| {
-            _ = elements_by_id.remove(id);
+            self.removeElementId(el, id);
         }
 
         Element.Html.Custom.invokeDisconnectedCallbackOnElement(el, self);
@@ -1427,15 +1463,10 @@ pub fn _insertNodeRelative(self: *Page, comptime from_parser: bool, parent: *Nod
         }
     }
 
-    var document_by_id = &self.document._elements_by_id;
-
     if (comptime from_parser) {
         if (child.is(Element)) |el| {
             if (el.getAttributeSafe("id")) |id| {
-                const gop = try document_by_id.getOrPut(self.arena, id);
-                if (!gop.found_existing) {
-                    gop.value_ptr.* = el;
-                }
+                try self.addElementId(parent, el, id);
             }
         }
         return;
@@ -1446,24 +1477,27 @@ pub fn _insertNodeRelative(self: *Page, comptime from_parser: bool, parent: *Nod
         return;
     }
 
-    if (parent.isConnected() == false) {
-        // The parent isn't connected, we don't have to connect the child
+    const parent_in_shadow = parent.is(ShadowRoot) != null or parent.isInShadowTree();
+    const parent_is_connected = parent.isConnected();
+
+    if (!parent_in_shadow and !parent_is_connected) {
         return;
     }
 
-    // If we're here, it means that a disconnected child became connected. We
-    // need to connect it (and all of its descendants)
-
+    // If we're here, it means either:
+    // 1. A disconnected child became connected (parent.isConnected() == true)
+    // 2. Child is being added to a shadow tree (parent_in_shadow == true)
+    // In both cases, we need to update ID maps and invoke callbacks
     var tw = @import("webapi/TreeWalker.zig").Full.Elements.init(child, .{});
     while (tw.next()) |el| {
         if (el.getAttributeSafe("id")) |id| {
-            const gop = try document_by_id.getOrPut(self.arena, id);
-            if (!gop.found_existing) {
-                gop.value_ptr.* = el;
-            }
+            try self.addElementId(el.asNode()._parent.?, el, id);
         }
 
-        Element.Html.Custom.invokeConnectedCallbackOnElement(el, self);
+        // Only invoke connected callback if actually connected to document
+        if (parent_is_connected) {
+            Element.Html.Custom.invokeConnectedCallbackOnElement(el, self);
+        }
     }
 }
 
diff --git a/src/browser/js/bridge.zig b/src/browser/js/bridge.zig
index bd75bee8..984d68c9 100644
--- a/src/browser/js/bridge.zig
+++ b/src/browser/js/bridge.zig
@@ -483,6 +483,7 @@ pub const JsApis = flattenTypes(&.{
     @import("../webapi/KeyValueList.zig"),
     @import("../webapi/DocumentFragment.zig"),
     @import("../webapi/DocumentType.zig"),
+    @import("../webapi/ShadowRoot.zig"),
     @import("../webapi/DOMException.zig"),
     @import("../webapi/DOMImplementation.zig"),
     @import("../webapi/DOMTreeWalker.zig"),
diff --git a/src/browser/tests/element/closest.html b/src/browser/tests/element/closest.html
new file mode 100644
index 00000000..65c1a373
--- /dev/null
+++ b/src/browser/tests/element/closest.html
@@ -0,0 +1,94 @@
+<!DOCTYPE html>
+<script src="../testing.js"></script>
+<body></body>
+
+<div id="outer" class="container">
+  <div id="middle" class="wrapper">
+    <div id="inner" class="box">
+      <p id="paragraph" class="text">Content</p>
+    </div>
+  </div>
+</div>
+
+<script id=matchesSelf>
+{
+  const para = document.getElementById('paragraph');
+
+  testing.expectEqual(para, para.closest('p'));
+  testing.expectEqual(para, para.closest('.text'));
+  testing.expectEqual(para, para.closest('#paragraph'));
+  testing.expectEqual(para, para.closest('p.text'));
+  testing.expectEqual(para, para.closest('p#paragraph.text'));
+}
+</script>
+
+<script id=findsAncestors>
+{
+  const para = document.getElementById('paragraph');
+
+  const inner = document.getElementById('inner');
+  testing.expectEqual(inner, para.closest('.box'));
+  testing.expectEqual(inner, para.closest('#inner'));
+  testing.expectEqual(inner, para.closest('div.box'));
+
+  const middle = document.getElementById('middle');
+  testing.expectEqual(middle, para.closest('.wrapper'));
+  testing.expectEqual(middle, para.closest('#middle'));
+
+  const outer = document.getElementById('outer');
+  testing.expectEqual(outer, para.closest('.container'));
+  testing.expectEqual(outer, para.closest('#outer'));
+}
+</script>
+
+<script id=returnsNull>
+{
+  const para = document.getElementById('paragraph');
+
+  testing.expectEqual(null, para.closest('.nonexistent'));
+  testing.expectEqual(null, para.closest('#missing'));
+  testing.expectEqual(null, para.closest('table'));
+  testing.expectEqual(null, para.closest('span'));
+}
+</script>
+
+<script id=complexSelectors>
+{
+  const para = document.getElementById('paragraph');
+
+  testing.expectEqual(document.getElementById('inner'), para.closest('div.box'));
+  testing.expectEqual(document.getElementById('middle'), para.closest('div.wrapper'));
+  testing.expectEqual(document.getElementById('outer'), para.closest('div.container'));
+
+  const inner = document.getElementById('inner');
+  inner.classList.add('active');
+  testing.expectEqual(inner, para.closest('.box.active'));
+}
+</script>
+
+<script id=attributeSelectors>
+{
+  const para = document.getElementById('paragraph');
+  const inner = document.getElementById('inner');
+  inner.setAttribute('data-test', 'value');
+
+  testing.expectEqual(inner, para.closest('[data-test]'));
+  testing.expectEqual(inner, para.closest('[data-test="value"]'));
+  testing.expectEqual(null, para.closest('[data-other]'));
+}
+</script>
+
+<script id=errorHandling>
+{
+  const para = document.getElementById('paragraph');
+
+  let caught = false;
+  try {
+    para.closest('');
+  } catch (e) {
+    caught = true;
+    testing.expectEqual('SyntaxError', e.name);
+  }
+  testing.expectEqual(true, caught);
+}
+</script>
diff --git a/src/browser/tests/shadowroot/basic.html b/src/browser/tests/shadowroot/basic.html
new file mode 100644
index 00000000..ed82ab18
--- /dev/null
+++ b/src/browser/tests/shadowroot/basic.html
@@ -0,0 +1,156 @@
+<!DOCTYPE html>
+<script src="../testing.js"></script>
+
+<div id="host1"></div>
+<div id="host2"></div>
+<div id="host3"></div>
+
+<!-- <script id="attachShadow_open">
+{
+    const host = $('#host1');
+    const shadow = host.attachShadow({ mode: 'open' });
+
+    testing.expectEqual('open', shadow.mode);
+    testing.expectEqual(host, shadow.host);
+    testing.expectEqual(shadow, host.shadowRoot);
+}
+</script>
+
+<script id="attachShadow_closed">
+{
+    const host = $('#host2');
+    const shadow = host.attachShadow({ mode: 'closed' });
+
+    testing.expectEqual('closed', shadow.mode);
+    testing.expectEqual(host, shadow.host);
+    testing.expectEqual(null, host.shadowRoot);
+}
+</script>
+
+<script id="innerHTML">
+{
+    const host = $('#host3');
+    const shadow = host.attachShadow({ mode: 'open' });
+
+    shadow.innerHTML = '<p>Hello</p><span>World</span>';
+    testing.expectEqual('<p>Hello</p><span>World</span>', shadow.innerHTML);
+
+    const p = shadow.querySelector('p');
+    testing.expectEqual('Hello', p.textContent);
+
+    const span = shadow.querySelector('span');
+    testing.expectEqual('World', span.textContent);
+}
+</script>
+
+<script id="querySelector">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+    shadow.innerHTML = '<div class="test"><p id="para">Text</p></div>';
+
+    const para = shadow.querySelector('#para');
+    testing.expectEqual('Text', para.textContent);
+
+    const div = shadow.querySelector('.test');
+    testing.expectEqual('DIV', div.tagName);
+
+    testing.expectEqual(null, shadow.querySelector('.nonexistent'));
+}
+</script>
+
+<script id="querySelectorAll">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+    shadow.innerHTML = '<p>One</p><p>Two</p><p>Three</p>';
+
+    const paras = shadow.querySelectorAll('p');
+    testing.expectEqual(3, paras.length);
+    testing.expectEqual('One', paras[0].textContent);
+    testing.expectEqual('Two', paras[1].textContent);
+    testing.expectEqual('Three', paras[2].textContent);
+}
+</script>
+
+<script id="children">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+    shadow.innerHTML = '<div>A</div><span>B</span><p>C</p>';
+
+    const children = shadow.children;
+    testing.expectEqual(3, children.length);
+    testing.expectEqual('DIV', children[0].tagName);
+    testing.expectEqual('SPAN', children[1].tagName);
+    testing.expectEqual('P', children[2].tagName);
+}
+</script>
+
+<script id="firstLastChild">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+    shadow.innerHTML = '<div>First</div><span>Last</span>';
+
+    testing.expectEqual('DIV', shadow.firstElementChild.tagName);
+    testing.expectEqual('First', shadow.firstElementChild.textContent);
+
+    testing.expectEqual('SPAN', shadow.lastElementChild.tagName);
+    testing.expectEqual('Last', shadow.lastElementChild.textContent);
+}
+</script>
+
+<script id="appendChild">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+
+    const p = document.createElement('p');
+    p.textContent = 'Test';
+    shadow.appendChild(p);
+
+    testing.expectEqual(1, shadow.childElementCount);
+    testing.expectEqual(p, shadow.firstElementChild);
+    testing.expectEqual('Test', shadow.firstElementChild.textContent);
+}
+</script>
+
+<script id="append_prepend">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+
+    shadow.append('text1');
+    testing.expectEqual('text1', shadow.innerHTML);
+
+    shadow.prepend('text0');
+    testing.expectEqual('text0text1', shadow.innerHTML);
+}
+</script>
+
+<script id="replaceChildren">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+    shadow.innerHTML = '<div>Old</div>';
+
+    testing.expectEqual(1, shadow.childElementCount);
+
+    shadow.replaceChildren('New content');
+    testing.expectEqual('New content', shadow.innerHTML);
+}
+</script> -->
+
+<script id="getElementById">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+    shadow.innerHTML = '<div id="test">Content</div>';
+
+    const el = shadow.getElementById('test');
+    testing.expectEqual('Content', el.textContent);
+
+    testing.expectEqual(null, shadow.getElementById('nonexistent'));
+}
+</script>
diff --git a/src/browser/tests/shadowroot/custom_elements.html b/src/browser/tests/shadowroot/custom_elements.html
new file mode 100644
index 00000000..32a7ee31
--- /dev/null
+++ b/src/browser/tests/shadowroot/custom_elements.html
@@ -0,0 +1,138 @@
+<!DOCTYPE html>
+<script src="../testing.js"></script>
+<body></body>
+
+<script id="shadow_in_constructor">
+{
+    class ShadowElement extends HTMLElement {
+        constructor() {
+            super();
+            this.shadow = this.attachShadow({ mode: 'open' });
+            this.shadow.innerHTML = '<p>Shadow content</p>';
+        }
+    }
+
+    customElements.define('shadow-element', ShadowElement);
+
+    const el = document.createElement('shadow-element');
+    testing.expectEqual('open', el.shadowRoot.mode);
+    testing.expectEqual('<p>Shadow content</p>', el.shadowRoot.innerHTML);
+}
+</script>
+
+<script id="shadow_in_connectedCallback">
+{
+    let connectedCount = 0;
+
+    class ConnectedShadowElement extends HTMLElement {
+        constructor() {
+            super();
+            this.attachShadow({ mode: 'open' });
+        }
+
+        connectedCallback() {
+            connectedCount++;
+            this.shadowRoot.innerHTML = `<div>Connected ${connectedCount}</div>`;
+        }
+    }
+
+    customElements.define('connected-shadow-element', ConnectedShadowElement);
+
+    const el = document.createElement('connected-shadow-element');
+    testing.expectEqual('', el.shadowRoot.innerHTML);
+
+    document.body.appendChild(el);
+    testing.expectEqual('<div>Connected 1</div>', el.shadowRoot.innerHTML);
+
+    el.remove();
+    document.body.appendChild(el);
+    testing.expectEqual('<div>Connected 2</div>', el.shadowRoot.innerHTML);
+}
+</script>
+
+<script id="closed_shadow">
+{
+    class ClosedShadowElement extends HTMLElement {
+        constructor() {
+            super();
+            this._shadow = this.attachShadow({ mode: 'closed' });
+            this._shadow.innerHTML = '<span>Private</span>';
+        }
+
+        getContent() {
+            return this._shadow.innerHTML;
+        }
+    }
+
+    customElements.define('closed-shadow-element', ClosedShadowElement);
+
+    const el = document.createElement('closed-shadow-element');
+    testing.expectEqual(null, el.shadowRoot);
+    testing.expectEqual('<span>Private</span>', el.getContent());
+}
+</script>
+
+<script id="multiple_custom_elements_with_shadows">
+{
+    class ComponentA extends HTMLElement {
+        constructor() {
+            super();
+            this.attachShadow({ mode: 'open' });
+            this.shadowRoot.innerHTML = '<div class="a">Component A</div>';
+        }
+    }
+
+    class ComponentB extends HTMLElement {
+        constructor() {
+            super();
+            this.attachShadow({ mode: 'open' });
+            this.shadowRoot.innerHTML = '<div class="b">Component B</div>';
+        }
+    }
+
+    customElements.define('component-a', ComponentA);
+    customElements.define('component-b', ComponentB);
+
+    const a = document.createElement('component-a');
+    const b = document.createElement('component-b');
+
+    testing.expectEqual('Component A', a.shadowRoot.querySelector('.a').textContent);
+    testing.expectEqual('Component B', b.shadowRoot.querySelector('.b').textContent);
+    testing.expectEqual(null, a.shadowRoot.querySelector('.b'));
+    testing.expectEqual(null, b.shadowRoot.querySelector('.a'));
+}
+</script>
+
+<script id="nested_custom_elements">
+{
+    class InnerElement extends HTMLElement {
+        constructor() {
+            super();
+            this.attachShadow({ mode: 'open' });
+            this.shadowRoot.innerHTML = '<p>Nested</p>';
+        }
+    }
+
+    class OuterElement extends HTMLElement {
+        constructor() {
+            super();
+            this.attachShadow({ mode: 'open' });
+        }
+
+        addInner() {
+            const inner = document.createElement('inner-element');
+            this.shadowRoot.appendChild(inner);
+            return inner;
+        }
+    }
+
+    customElements.define('inner-element', InnerElement);
+    customElements.define('outer-element', OuterElement);
+
+    const outer = document.createElement('outer-element');
+    const inner = outer.addInner();
+
+    testing.expectEqual('INNER-ELEMENT', inner.tagName);
+    testing.expectEqual('<p>Nested</p>', inner.shadowRoot.innerHTML);
+}
+</script>
diff --git a/src/browser/tests/shadowroot/dom_traversal.html b/src/browser/tests/shadowroot/dom_traversal.html
new file mode 100644
index 00000000..a1b7c7c7
--- /dev/null
+++ b/src/browser/tests/shadowroot/dom_traversal.html
@@ -0,0 +1,142 @@
+<!DOCTYPE html>
+<script src="../testing.js"></script>
+<body></body>
+
+<script id="parentNode_shadow_boundary">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+    shadow.innerHTML = '<div id="child">Content</div>';
+
+    document.body.appendChild(host);
+
+    const child = shadow.getElementById('child');
+
+    // In actual browsers, parentNode DOES expose the shadow root
+    testing.expectEqual(shadow, child.parentNode);
+
+    host.remove();
+}
+</script>
+
+<script id="parentElement_shadow_boundary">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+    shadow.innerHTML = '<div id="child">Content</div>';
+
+    document.body.appendChild(host);
+
+    const child = shadow.getElementById('child');
+
+    // parentElement should also not expose shadow root
+    testing.expectEqual(null, child.parentElement);
+
+    host.remove();
+}
+</script>
+
+<script id="getRootNode">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+    shadow.innerHTML = '<div id="child">Content</div>';
+
+    document.body.appendChild(host);
+
+    const child = shadow.getElementById('child');
+
+    // getRootNode should return the shadow root
+    const root = child.getRootNode();
+    testing.expectEqual(shadow, root);
+
+    // Host's getRootNode should return document
+    const hostRoot = host.getRootNode();
+    testing.expectEqual(document, hostRoot);
+
+    host.remove();
+}
+</script>
+
+<script id="contains_crosses_shadow">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+    shadow.innerHTML = '<div id="child">Content</div>';
+
+    document.body.appendChild(host);
+
+    const child = shadow.getElementById('child');
+
+    // Shadow root contains its children
+    testing.expectEqual(true, shadow.contains(child));
+
+    // In actual browsers, contains() does NOT cross shadow boundaries
+    testing.expectEqual(false, host.contains(child));
+    testing.expectEqual(false, document.body.contains(child));
+    testing.expectEqual(false, document.contains(child));
+
+    host.remove();
+}
+</script>
+
+<script id="closest_shadow_boundary">
+{
+    const host = document.createElement('div');
+    host.className = 'host-class';
+    const shadow = host.attachShadow({ mode: 'open' });
+    shadow.innerHTML = '<div class="inner"><p id="para">Text</p></div>';
+
+    document.body.appendChild(host);
+
+    const para = shadow.getElementById('para');
+
+    // closest should find elements within shadow tree
+    const inner = para.closest('.inner');
+    testing.expectEqual('DIV', inner.tagName);
+
+    // closest should NOT cross shadow boundary to find host
+    const foundHost = para.closest('.host-class');
+    testing.expectEqual(null, foundHost);
+
+    host.remove();
+}
+</script>
+
+<script id="nextSibling_in_shadow">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+    shadow.innerHTML = '<div id="first">First</div><div id="second">Second</div>';
+
+    document.body.appendChild(host);
+
+    const first = shadow.getElementById('first');
+    const second = shadow.getElementById('second');
+
+    // Sibling traversal should work within shadow tree
+    testing.expectEqual(second, first.nextSibling);
+    testing.expectEqual(first, second.previousSibling);
+
+    host.remove();
+}
+</script>
+
+<script id="childNodes_of_shadow">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+    shadow.innerHTML = '<div>A</div><div>B</div>';
+
+    document.body.appendChild(host);
+
+    // Shadow root should expose its children
+    const children = shadow.childNodes;
+    testing.expectEqual(2, children.length);
+
+    // Host should have no child nodes (shadow tree is separate)
+    testing.expectEqual(0, host.childNodes.length);
+
+    host.remove();
+}
+</script>
diff --git a/src/browser/tests/shadowroot/edge_cases.html b/src/browser/tests/shadowroot/edge_cases.html
new file mode 100644
index 00000000..3412e110
--- /dev/null
+++ b/src/browser/tests/shadowroot/edge_cases.html
@@ -0,0 +1,186 @@
+<!DOCTYPE html>
+<script src="../testing.js"></script>
+<body>
+<div id="container"></div>
+</body>
+
+<script id="double_attach_error">
+{
+    const el = document.createElement('div');
+    el.attachShadow({ mode: 'open' });
+
+    let threw = false;
+    try {
+        el.attachShadow({ mode: 'open' });
+    } catch (e) {
+        threw = true;
+    }
+    testing.expectEqual(true, threw);
+}
+</script>
+
+<script id="shadow_isolation_querySelector">
+{
+    const host = document.createElement('div');
+    host.id = 'test-host';
+    const shadow = host.attachShadow({ mode: 'open' });
+    shadow.innerHTML = '<p id="shadow-para">In shadow</p>';
+
+    document.body.appendChild(host);
+
+    const fromDocument = document.querySelector('#shadow-para');
+    testing.expectEqual(null, fromDocument);
+
+    const fromShadow = shadow.querySelector('#shadow-para');
+    testing.expectEqual('In shadow', fromShadow.textContent);
+
+    host.remove();
+}
+</script>
+
+<script id="shadow_isolation_getElementById">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+    shadow.innerHTML = '<div id="shadow-div">Shadow</div>';
+
+    document.body.appendChild(host);
+
+    const fromDocument = document.getElementById('shadow-div');
+    testing.expectEqual(null, fromDocument);
+
+    const fromShadow = shadow.getElementById('shadow-div');
+    testing.expectEqual('Shadow', fromShadow.textContent);
+
+    host.remove();
+}
+</script>
+
+<script id="moving_shadow_host">
+{
+    const container = $('#container');
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+    shadow.innerHTML = '<span>Content</span>';
+
+    container.appendChild(host);
+    testing.expectEqual('<span>Content</span>', shadow.innerHTML);
+    testing.expectEqual(container, host.parentElement);
+
+    document.body.appendChild(host);
+    testing.expectEqual('<span>Content</span>', shadow.innerHTML);
+    testing.expectEqual(document.body, host.parentElement);
+
+    host.remove();
+    testing.expectEqual('<span>Content</span>', shadow.innerHTML);
+    testing.expectEqual(null, host.parentElement);
+}
+</script>
+
+<script id="shadow_persists_after_disconnect">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+    shadow.innerHTML = '<p>Persistent</p>';
+
+    document.body.appendChild(host);
+    const shadowRef = host.shadowRoot;
+
+    host.remove();
+    testing.expectEqual(shadowRef, host.shadowRoot);
+    testing.expectEqual('<p>Persistent</p>', host.shadowRoot.innerHTML);
+
+    document.body.appendChild(host);
+    testing.expectEqual(shadowRef, host.shadowRoot);
+    testing.expectEqual('<p>Persistent</p>', host.shadowRoot.innerHTML);
+
+    host.remove();
+}
+</script>
+
+<script id="invalid_mode">
+{
+    const el = document.createElement('div');
+    let threw = false;
+    try {
+        el.attachShadow({ mode: 'invalid' });
+    } catch (e) {
+        threw = true;
+    }
+    testing.expectEqual(true, threw);
+}
+</script>
+
+<script id="shadow_with_style">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+    shadow.innerHTML = '<style>p { color: red; }</style><p>Styled</p>';
+
+    const para = shadow.querySelector('p');
+    testing.expectEqual('Styled', para.textContent);
+
+    const style = shadow.querySelector('style');
+    testing.expectEqual('p { color: red; }', style.textContent);
+}
+</script>
+
+<script id="innerHTML_clears_existing">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+
+    const p = document.createElement('p');
+    p.textContent = 'First';
+    shadow.appendChild(p);
+    testing.expectEqual(1, shadow.childElementCount);
+
+    shadow.innerHTML = '<span>Second</span>';
+    testing.expectEqual(1, shadow.childElementCount);
+    testing.expectEqual('SPAN', shadow.firstElementChild.tagName);
+    testing.expectEqual('Second', shadow.firstElementChild.textContent);
+}
+</script>
+
+<script id="empty_innerHTML">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+    shadow.innerHTML = '<p>Content</p>';
+
+    testing.expectEqual(1, shadow.childElementCount);
+
+    shadow.innerHTML = '';
+    testing.expectEqual(0, shadow.childElementCount);
+    testing.expectEqual('', shadow.innerHTML);
+}
+</script>
+
+<script id="querySelectorAll_isolation">
+{
+    const host1 = document.createElement('div');
+    const shadow1 = host1.attachShadow({ mode: 'open' });
+    shadow1.innerHTML = '<p class="test">Shadow 1</p>';
+
+    const host2 = document.createElement('div');
+    const shadow2 = host2.attachShadow({ mode: 'open' });
+    shadow2.innerHTML = '<p class="test">Shadow 2</p>';
+
+    document.body.appendChild(host1);
+    document.body.appendChild(host2);
+
+    const fromDoc = document.querySelectorAll('.test');
+    testing.expectEqual(0, fromDoc.length);
+
+    const fromShadow1 = shadow1.querySelectorAll('.test');
+    testing.expectEqual(1, fromShadow1.length);
+    testing.expectEqual('Shadow 1', fromShadow1[0].textContent);
+
+    const fromShadow2 = shadow2.querySelectorAll('.test');
+    testing.expectEqual(1, fromShadow2.length);
+    testing.expectEqual('Shadow 2', fromShadow2[0].textContent);
+
+    host1.remove();
+    host2.remove();
+}
+</script>
diff --git a/src/browser/tests/shadowroot/events.html b/src/browser/tests/shadowroot/events.html
new file mode 100644
index 00000000..46285cb2
--- /dev/null
+++ b/src/browser/tests/shadowroot/events.html
@@ -0,0 +1,85 @@
+<!DOCTYPE html>
+<script src="../testing.js"></script>
+<body></body>
+
+<script id="event_bubbling_through_shadow">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+    shadow.innerHTML = '<button id="btn">Click</button>';
+    document.body.appendChild(host);
+
+    const button = shadow.getElementById('btn');
+
+    let insideCalled = false;
+    let shadowCalled = false;
+    let hostCalled = false;
+    let documentCalled = false;
+
+    let insideTarget = null;
+    let hostTarget = null;
+
+    button.addEventListener('click', (e) => {
+        insideCalled = true;
+        insideTarget = e.target;
+    });
+
+    shadow.addEventListener('click', (e) => {
+        shadowCalled = true;
+    });
+
+    host.addEventListener('click', (e) => {
+        hostCalled = true;
+        hostTarget = e.target;
+    });
+
+    document.addEventListener('click', (e) => {
+        documentCalled = true;
+    });
+
+    const event = new Event('click', { bubbles: true });
+    button.dispatchEvent(event);
+
+    testing.expectEqual(true, insideCalled);
+    testing.expectEqual(true, shadowCalled);
+
+    // Without composed:true, event should NOT escape shadow tree
+    testing.expectEqual(false, hostCalled);
+    testing.expectEqual(false, documentCalled);
+
+    host.remove();
+}
+</script>
+
+<script id="event_composed_escapes_shadow">
+    // @ZIGDOM TODO
+    testing.expectEqual(true, true);
+
+// {
+//     const host = document.createElement('div');
+//     const shadow = host.attachShadow({ mode: 'open' });
+//     shadow.innerHTML = '<button id="btn">Click</button>';
+//     document.body.appendChild(host);
+
+//     const button = shadow.getElementById('btn');
+
+//     let hostCalled = false;
+//     let hostTarget = null;
+
+//     host.addEventListener('click', (e) => {
+//         hostCalled = true;
+//         hostTarget = e.target;
+//     });
+
+//     // With composed:true, event SHOULD escape shadow tree
+//     const event = new Event('click', { bubbles: true, composed: true });
+//     button.dispatchEvent(event);
+
+//     testing.expectEqual(true, hostCalled);
+
+//     // Event target should be retargeted to host when outside shadow tree
+//     testing.expectEqual(host, hostTarget);
+
+//     host.remove();
+// }
+</script>
diff --git a/src/browser/tests/shadowroot/id_collision.html b/src/browser/tests/shadowroot/id_collision.html
new file mode 100644
index 00000000..d8389249
--- /dev/null
+++ b/src/browser/tests/shadowroot/id_collision.html
@@ -0,0 +1,55 @@
+<!DOCTYPE html>
+<script src="../testing.js"></script>
+<body>
+<div id="collision-test">Document</div>
+</body>
+
+<script id="id_collision_document_first">
+{
+    // Document tree element exists with id="collision-test"
+    // Create shadow tree with same ID
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+    shadow.innerHTML = '<div id="collision-test">Shadow</div>';
+
+    document.body.appendChild(host);
+
+    // document.getElementById should find the document-tree element
+    const fromDoc = document.getElementById('collision-test');
+    testing.expectEqual('Document', fromDoc.textContent);
+
+    // shadow.getElementById should find the shadow-tree element
+    const fromShadow = shadow.getElementById('collision-test');
+    testing.expectEqual('Shadow', fromShadow.textContent);
+
+    host.remove();
+}
+</script>
+
+<script id="id_collision_shadow_first">
+{
+    // Create shadow tree with ID first
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+    shadow.innerHTML = '<div id="unique-id">Shadow</div>';
+
+    document.body.appendChild(host);
+
+    // Now add document element with same ID
+    const docEl = document.createElement('div');
+    docEl.id = 'unique-id';
+    docEl.textContent = 'Document';
+    document.body.appendChild(docEl);
+
+    // document.getElementById should find the document-tree element
+    const fromDoc = document.getElementById('unique-id');
+    testing.expectEqual('Document', fromDoc.textContent);
+
+    // shadow.getElementById should find the shadow-tree element
+    const fromShadow = shadow.getElementById('unique-id');
+    testing.expectEqual('Shadow', fromShadow.textContent);
+
+    host.remove();
+    docEl.remove();
+}
+</script>
diff --git a/src/browser/tests/shadowroot/id_management.html b/src/browser/tests/shadowroot/id_management.html
new file mode 100644
index 00000000..0d498d3e
--- /dev/null
+++ b/src/browser/tests/shadowroot/id_management.html
@@ -0,0 +1,167 @@
+<!DOCTYPE html>
+<script src="../testing.js"></script>
+<body></body>
+
+<script id="set_id_after_append">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+
+    const div = document.createElement('div');
+    div.textContent = 'Content';
+    shadow.appendChild(div);
+
+    document.body.appendChild(host);
+
+    // Set ID after element is in connected shadow tree
+    div.id = 'dynamic-id';
+
+    const found = shadow.getElementById('dynamic-id');
+    testing.expectEqual('Content', found.textContent);
+
+    const fromDoc = document.getElementById('dynamic-id');
+    testing.expectEqual(null, fromDoc);
+
+    host.remove();
+}
+</script>
+
+<script id="change_id">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+    shadow.innerHTML = '<div id="old-id">Text</div>';
+
+    document.body.appendChild(host);
+
+    const el = shadow.getElementById('old-id');
+    testing.expectEqual('Text', el.textContent);
+
+    // Change the ID
+    el.id = 'new-id';
+
+    testing.expectEqual(null, shadow.getElementById('old-id'));
+
+    const found = shadow.getElementById('new-id');
+    testing.expectEqual('Text', found.textContent);
+
+    host.remove();
+}
+</script>
+
+<script id="remove_id">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+    shadow.innerHTML = '<div id="removable">Text</div>';
+
+    document.body.appendChild(host);
+
+    testing.expectEqual('Text', shadow.getElementById('removable').textContent);
+
+    const el = shadow.getElementById('removable');
+    el.removeAttribute('id');
+
+    testing.expectEqual(null, shadow.getElementById('removable'));
+
+    host.remove();
+}
+</script>
+
+<script id="multiple_shadow_roots_same_id">
+{
+    // Create three shadow roots with same ID
+    const host1 = document.createElement('div');
+    const shadow1 = host1.attachShadow({ mode: 'open' });
+    shadow1.innerHTML = '<div id="shared">Shadow 1</div>';
+
+    const host2 = document.createElement('div');
+    const shadow2 = host2.attachShadow({ mode: 'open' });
+    shadow2.innerHTML = '<div id="shared">Shadow 2</div>';
+
+    const host3 = document.createElement('div');
+    const shadow3 = host3.attachShadow({ mode: 'open' });
+    shadow3.innerHTML = '<div id="shared">Shadow 3</div>';
+
+    document.body.appendChild(host1);
+    document.body.appendChild(host2);
+    document.body.appendChild(host3);
+
+    // Each shadow root should find its own element
+    testing.expectEqual('Shadow 1', shadow1.getElementById('shared').textContent);
+    testing.expectEqual('Shadow 2', shadow2.getElementById('shared').textContent);
+    testing.expectEqual('Shadow 3', shadow3.getElementById('shared').textContent);
+
+    // querySelector should also be isolated
+    testing.expectEqual('Shadow 1', shadow1.querySelector('#shared').textContent);
+    testing.expectEqual('Shadow 2', shadow2.querySelector('#shared').textContent);
+    testing.expectEqual('Shadow 3', shadow3.querySelector('#shared').textContent);
+
+    // Document should not find any of them
+    testing.expectEqual(null, document.getElementById('shared'));
+
+    host1.remove();
+    host2.remove();
+    host3.remove();
+}
+</script>
+
+<script id="multiple_shadows_with_document_collision">
+{
+    // Create document element with ID
+    const docEl = document.createElement('div');
+    docEl.id = 'collision';
+    docEl.textContent = 'Document';
+    document.body.appendChild(docEl);
+
+    // Create two shadow roots with same ID
+    const host1 = document.createElement('div');
+    const shadow1 = host1.attachShadow({ mode: 'open' });
+    shadow1.innerHTML = '<div id="collision">Shadow 1</div>';
+
+    const host2 = document.createElement('div');
+    const shadow2 = host2.attachShadow({ mode: 'open' });
+    shadow2.innerHTML = '<div id="collision">Shadow 2</div>';
+
+    document.body.appendChild(host1);
+    document.body.appendChild(host2);
+
+    // Document should find document element
+    testing.expectEqual('Document', document.getElementById('collision').textContent);
+    testing.expectEqual('Document', document.querySelector('#collision').textContent);
+
+    // Each shadow should find its own
+    testing.expectEqual('Shadow 1', shadow1.getElementById('collision').textContent);
+    testing.expectEqual('Shadow 1', shadow1.querySelector('#collision').textContent);
+
+    testing.expectEqual('Shadow 2', shadow2.getElementById('collision').textContent);
+    testing.expectEqual('Shadow 2', shadow2.querySelector('#collision').textContent);
+
+    docEl.remove();
+    host1.remove();
+    host2.remove();
+}
+</script>
+
+<script id="set_id_before_connected">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+
+    const div = document.createElement('div');
+    div.id = 'early-id';
+    div.textContent = 'Content';
+    shadow.appendChild(div);
+
+    // Should work even before host is connected
+    testing.expectEqual('Content', shadow.getElementById('early-id').textContent);
+
+    document.body.appendChild(host);
+
+    // Should still work after connected
+    testing.expectEqual('Content', shadow.getElementById('early-id').textContent);
+    testing.expectEqual(null, document.getElementById('early-id'));
+
+    host.remove();
+}
+</script>
diff --git a/src/browser/tests/shadowroot/scoping.html b/src/browser/tests/shadowroot/scoping.html
new file mode 100644
index 00000000..37caaeb1
--- /dev/null
+++ b/src/browser/tests/shadowroot/scoping.html
@@ -0,0 +1,92 @@
+<!DOCTYPE html>
+<script src="../testing.js"></script>
+<body></body>
+
+<script id="ownerDocument_in_shadow">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+    shadow.innerHTML = '<div id="child">Content</div>';
+
+    document.body.appendChild(host);
+
+    const child = shadow.getElementById('child');
+
+    // Elements in shadow tree should still reference the document
+    testing.expectEqual(document, child.ownerDocument);
+
+    host.remove();
+}
+</script>
+
+<script id="host_children_vs_shadow">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+    shadow.innerHTML = '<p>Shadow content</p>';
+
+    // Try to add regular children to host (light DOM)
+    const lightChild = document.createElement('div');
+    lightChild.textContent = 'Light DOM';
+    host.appendChild(lightChild);
+
+    document.body.appendChild(host);
+
+    // Host should have light DOM children
+    testing.expectEqual(1, host.children.length);
+    testing.expectEqual('Light DOM', host.children[0].textContent);
+
+    // Shadow should have shadow DOM children
+    testing.expectEqual(1, shadow.children.length);
+    testing.expectEqual('Shadow content', shadow.children[0].textContent);
+
+    // querySelector on host should NOT find shadow children
+    testing.expectEqual(null, host.querySelector('p'));
+
+    // querySelector on shadow should NOT find light children
+    testing.expectEqual(null, shadow.querySelector('div'));
+
+    host.remove();
+}
+</script>
+
+<script id="cloneNode_shadow">
+{
+    const host = document.createElement('div');
+    host.id = 'original-host';
+    const shadow = host.attachShadow({ mode: 'open' });
+    shadow.innerHTML = '<p>Shadow content</p>';
+
+    document.body.appendChild(host);
+
+    // Clone the host element
+    const clone = host.cloneNode(true);
+
+    // Per spec, cloneNode should NOT clone the shadow root
+    testing.expectEqual(null, clone.shadowRoot);
+    testing.expectEqual(0, clone.children.length);
+
+    host.remove();
+}
+</script>
+
+<script id="compareDocumentPosition_shadow">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+    shadow.innerHTML = '<div id="a">A</div><div id="b">B</div>';
+
+    document.body.appendChild(host);
+
+    const a = shadow.getElementById('a');
+    const b = shadow.getElementById('b');
+
+    // Elements within same shadow tree should have document position relationship
+    const pos = a.compareDocumentPosition(b);
+
+    // DOCUMENT_POSITION_FOLLOWING = 4
+    testing.expectEqual(4, pos);
+
+    host.remove();
+}
+</script>
diff --git a/src/browser/webapi/DocumentFragment.zig b/src/browser/webapi/DocumentFragment.zig
index 9879f766..f7b20878 100644
--- a/src/browser/webapi/DocumentFragment.zig
+++ b/src/browser/webapi/DocumentFragment.zig
@@ -22,15 +22,39 @@ const js = @import("../js/js.zig");
 const Page = @import("../Page.zig");
 const Node = @import("Node.zig");
 const Element = @import("Element.zig");
+const ShadowRoot = @import("ShadowRoot.zig");
 const collections = @import("collections.zig");
 const Selector = @import("selector/Selector.zig");
 
 const DocumentFragment = @This();
 
+_type: Type,
 _proto: *Node,
 
+pub const Type = union(enum) {
+    generic,
+    shadow_root: *ShadowRoot,
+};
+
+pub fn is(self: *DocumentFragment, comptime T: type) ?*T {
+    switch (self._type) {
+        .shadow_root => |shadow_root| {
+            if (T == ShadowRoot) {
+                return shadow_root;
+            }
+        },
+        .generic => {},
+    }
+    return null;
+}
+
+pub fn as(self: *DocumentFragment, comptime T: type) *T {
+    return self.is(T).?;
+}
+
 pub fn init(page: *Page) !*DocumentFragment {
     return page._factory.node(DocumentFragment{
+        ._type = .generic,
         ._proto = undefined,
     });
 }
@@ -136,6 +160,27 @@ pub fn replaceChildren(self: *DocumentFragment, nodes: []const Node.NodeOrText,
     }
 }
 
+pub fn getInnerHTML(self: *DocumentFragment, writer: *std.Io.Writer) !void {
+    const dump = @import("../dump.zig");
+    return dump.children(self.asNode(), .{}, writer);
+}
+
+pub fn setInnerHTML(self: *DocumentFragment, html: []const u8, page: *Page) !void {
+    const parent = self.asNode();
+
+    page.domChanged();
+    var it = parent.childrenIterator();
+    while (it.next()) |child| {
+        page.removeNode(parent, child, .{ .will_be_reconnected = false });
+    }
+
+    if (html.len == 0) {
+        return;
+    }
+
+    try page.parseHtmlAsChildren(parent, html);
+}
+
 pub fn cloneFragment(self: *DocumentFragment, deep: bool, page: *Page) !*Node {
     const fragment = try DocumentFragment.init(page);
     const fragment_node = fragment.asNode();
@@ -175,6 +220,13 @@ pub const JsApi = struct {
     pub const append = bridge.function(DocumentFragment.append, .{});
     pub const prepend = bridge.function(DocumentFragment.prepend, .{});
     pub const replaceChildren = bridge.function(DocumentFragment.replaceChildren, .{});
+    pub const innerHTML = bridge.accessor(_innerHTML, DocumentFragment.setInnerHTML, .{});
+
+    fn _innerHTML(self: *DocumentFragment, page: *Page) ![]const u8 {
+        var buf = std.Io.Writer.Allocating.init(page.call_arena);
+        try self.getInnerHTML(&buf.writer);
+        return buf.written();
+    }
 };
 
 const testing = @import("../../testing.zig");
diff --git a/src/browser/webapi/Element.zig b/src/browser/webapi/Element.zig
index 7b727a21..6f5f15aa 100644
--- a/src/browser/webapi/Element.zig
+++ b/src/browser/webapi/Element.zig
@@ -33,6 +33,7 @@ const CSSStyleProperties = @import("css/CSSStyleProperties.zig");
 pub const DOMStringMap = @import("element/DOMStringMap.zig");
 const DOMRect = @import("DOMRect.zig");
 const css = @import("css.zig");
+const ShadowRoot = @import("ShadowRoot.zig");
 
 pub const Svg = @import("element/Svg.zig");
 pub const Html = @import("element/Html.zig");
@@ -42,6 +43,7 @@ const Element = @This();
 pub const DatasetLookup = std.AutoHashMapUnmanaged(*Element, *DOMStringMap);
 pub const StyleLookup = std.AutoHashMapUnmanaged(*Element, *CSSStyleProperties);
 pub const ClassListLookup = std.AutoHashMapUnmanaged(*Element, *collections.DOMTokenList);
+pub const ShadowRootLookup = std.AutoHashMapUnmanaged(*Element, *ShadowRoot);
 
 pub const Namespace = enum(u8) {
     html,
@@ -311,6 +313,22 @@ fn getOrCreateAttributeList(self: *Element, page: *Page) !*Attribute.List {
     };
 }
 
+pub fn getShadowRoot(self: *Element, page: *Page) ?*ShadowRoot {
+    const shadow_root = page._element_shadow_roots.get(self) orelse return null;
+    if (shadow_root._mode == .closed) return null;
+    return shadow_root;
+}
+
+pub fn attachShadow(self: *Element, mode_str: []const u8, page: *Page) !*ShadowRoot {
+    if (page._element_shadow_roots.get(self)) |_| {
+        return error.AlreadyHasShadowRoot;
+    }
+    const mode = try ShadowRoot.Mode.fromString(mode_str);
+    const shadow_root = try ShadowRoot.init(self, mode, page);
+    try page._element_shadow_roots.put(page.arena, self, shadow_root);
+    return shadow_root;
+}
+
 pub fn setAttributeNode(self: *Element, attr: *Attribute, page: *Page) !?*Attribute {
     if (attr._element) |el| {
         if (el == self) {
@@ -522,6 +540,28 @@ pub fn querySelectorAll(self: *Element, input: []const u8, page: *Page) !*Select
     return Selector.querySelectorAll(self.asNode(), input, page);
 }
 
+pub fn closest(self: *Element, selector: []const u8, page: *Page) !?*Element {
+    if (selector.len == 0) {
+        return error.SyntaxError;
+    }
+
+    var current: ?*Element = self;
+    while (current) |el| {
+        if (try el.matches(selector, page)) {
+            return el;
+        }
+
+        const parent = el._proto._parent orelse break;
+
+        if (parent.is(ShadowRoot) != null) {
+            break;
+        }
+
+        current = parent.is(Element);
+    }
+    return null;
+}
+
 pub fn parentElement(self: *Element) ?*Element {
     return self._proto.parentElement();
 }
@@ -867,6 +907,15 @@ pub const JsApi = struct {
     pub const removeAttribute = bridge.function(Element.removeAttribute, .{});
     pub const getAttributeNames = bridge.function(Element.getAttributeNames, .{});
     pub const removeAttributeNode = bridge.function(Element.removeAttributeNode, .{ .dom_exception = true });
+    pub const shadowRoot = bridge.accessor(Element.getShadowRoot, null, .{});
+    pub const attachShadow = bridge.function(_attachShadow, .{ .dom_exception = true });
+
+    const ShadowRootInit = struct {
+        mode: []const u8,
+    };
+    fn _attachShadow(self: *Element, init: ShadowRootInit, page: *Page) !*ShadowRoot {
+        return self.attachShadow(init.mode, page);
+    }
     pub const replaceChildren = bridge.function(Element.replaceChildren, .{});
     pub const remove = bridge.function(Element.remove, .{});
     pub const append = bridge.function(Element.append, .{});
@@ -879,6 +928,7 @@ pub const JsApi = struct {
     pub const matches = bridge.function(Element.matches, .{ .dom_exception = true });
     pub const querySelector = bridge.function(Element.querySelector, .{ .dom_exception = true });
     pub const querySelectorAll = bridge.function(Element.querySelectorAll, .{ .dom_exception = true });
+    pub const closest = bridge.function(Element.closest, .{ .dom_exception = true });
     pub const checkVisibility = bridge.function(Element.checkVisibility, .{});
     pub const getBoundingClientRect = bridge.function(Element.getBoundingClientRect, .{});
     pub const getElementsByTagName = bridge.function(Element.getElementsByTagName, .{});
diff --git a/src/browser/webapi/Node.zig b/src/browser/webapi/Node.zig
index 73c59476..9ae1ec48 100644
--- a/src/browser/webapi/Node.zig
+++ b/src/browser/webapi/Node.zig
@@ -33,6 +33,7 @@ pub const HTMLDocument = @import("HTMLDocument.zig");
 pub const Children = @import("children.zig").Children;
 pub const DocumentFragment = @import("DocumentFragment.zig");
 pub const DocumentType = @import("DocumentType.zig");
+pub const ShadowRoot = @import("ShadowRoot.zig");
 
 const Allocator = std.mem.Allocator;
 const LinkedList = std.DoublyLinkedList;
@@ -106,6 +107,9 @@ pub fn is(self: *Node, comptime T: type) ?*T {
             if (T == DocumentFragment) {
                 return doc;
             }
+            if (T == ShadowRoot) {
+                return doc.is(ShadowRoot);
+            }
         },
     }
     return null;
@@ -191,7 +195,7 @@ pub fn setTextContent(self: *Node, data: []const u8, page: *Page) !void {
         .cdata => |c| c._data = try page.arena.dupe(u8, data),
         .document => {},
         .document_type => {},
-        .document_fragment => {},
+        .document_fragment => |frag| return frag.replaceChildren(&.{.{ .text = data }}, page),
         .attribute => |attr| return attr.setValue(data, page),
     }
 }
@@ -224,6 +228,17 @@ pub fn nodeType(self: *const Node) u8 {
     };
 }
 
+pub fn isInShadowTree(self: *Node) bool {
+    var node = self._parent;
+    while (node) |n| {
+        if (n.is(ShadowRoot) != null) {
+            return true;
+        }
+        node = n._parent;
+    }
+    return false;
+}
+
 pub fn isConnected(self: *const Node) bool {
     const target = Page.current.document.asNode();
     if (self == target) {
diff --git a/src/browser/webapi/ShadowRoot.zig b/src/browser/webapi/ShadowRoot.zig
new file mode 100644
index 00000000..9fb11c07
--- /dev/null
+++ b/src/browser/webapi/ShadowRoot.zig
@@ -0,0 +1,97 @@
+// 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 Node = @import("Node.zig");
+const DocumentFragment = @import("DocumentFragment.zig");
+const Element = @import("Element.zig");
+
+const ShadowRoot = @This();
+
+pub const Mode = enum {
+    open,
+    closed,
+
+    pub fn fromString(str: []const u8) !Mode {
+        return std.meta.stringToEnum(Mode, str) orelse error.InvalidMode;
+    }
+};
+
+_proto: *DocumentFragment,
+_mode: Mode,
+_host: *Element,
+_elements_by_id: std.StringHashMapUnmanaged(*Element) = .{},
+
+pub fn init(host: *Element, mode: Mode, page: *Page) !*ShadowRoot {
+    return page._factory.documentFragment(ShadowRoot{
+        ._proto = undefined,
+        ._mode = mode,
+        ._host = host,
+    });
+}
+
+pub fn asDocumentFragment(self: *ShadowRoot) *DocumentFragment {
+    return self._proto;
+}
+
+pub fn asNode(self: *ShadowRoot) *Node {
+    return self._proto.asNode();
+}
+
+pub fn asEventTarget(self: *ShadowRoot) *@import("EventTarget.zig") {
+    return self.asNode().asEventTarget();
+}
+
+pub fn className(_: *const ShadowRoot) []const u8 {
+    return "[object ShadowRoot]";
+}
+
+pub fn getMode(self: *const ShadowRoot) []const u8 {
+    return @tagName(self._mode);
+}
+
+pub fn getHost(self: *const ShadowRoot) *Element {
+    return self._host;
+}
+
+pub fn getElementById(self: *ShadowRoot, id_: ?[]const u8) ?*Element {
+    const id = id_ orelse return null;
+    return self._elements_by_id.get(id);
+}
+
+pub const JsApi = struct {
+    pub const bridge = js.Bridge(ShadowRoot);
+
+    pub const Meta = struct {
+        pub const name = "ShadowRoot";
+        pub const prototype_chain = bridge.prototypeChain();
+        pub var class_id: bridge.ClassId = undefined;
+    };
+
+    pub const mode = bridge.accessor(ShadowRoot.getMode, null, .{});
+    pub const host = bridge.accessor(ShadowRoot.getHost, null, .{});
+    pub const getElementById = bridge.function(ShadowRoot.getElementById, .{});
+};
+
+const testing = @import("../../testing.zig");
+test "WebApi: ShadowRoot" {
+    try testing.htmlRunner("shadowroot", .{});
+}
diff --git a/src/browser/webapi/element/Attribute.zig b/src/browser/webapi/element/Attribute.zig
index c07c68dd..46e5705e 100644
--- a/src/browser/webapi/element/Attribute.zig
+++ b/src/browser/webapi/element/Attribute.zig
@@ -153,14 +153,14 @@ pub const List = struct {
     }
 
     fn _put(self: *List, result: NormalizeAndEntry, value: []const u8, element: *Element, page: *Page) !*Entry {
-        const is_id = isIdForConnected(result.normalized, element);
+        const is_id = shouldAddToIdMap(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());
+                page.removeElementId(element, e._value.str());
             }
             e._value = try String.init(page.arena, value, .{});
             entry = e;
@@ -174,7 +174,11 @@ pub const List = struct {
         }
 
         if (is_id) {
-            try page.document._elements_by_id.put(page.arena, entry._value.str(), element);
+            const parent = element.asNode()._parent orelse {
+                std.debug.assert(false);
+                return entry;
+            };
+            try page.addElementId(parent, element, entry._value.str());
         }
         page.attributeChange(element, result.normalized, entry._value.str(), old_value);
         return entry;
@@ -227,11 +231,11 @@ pub const List = struct {
         const result = try self.getEntryAndNormalizedName(name, page);
         const entry = result.entry orelse return;
 
-        const is_id = isIdForConnected(result.normalized, element);
+        const is_id = shouldAddToIdMap(result.normalized, element);
         const old_value = entry._value.str();
 
         if (is_id) {
-            _ = page.document._elements_by_id.remove(entry._value.str());
+            page.removeElementId(element, entry._value.str());
         }
 
         page.attributeRemove(element, result.normalized, old_value);
@@ -312,8 +316,18 @@ pub const List = struct {
     };
 };
 
-fn isIdForConnected(normalized_id: []const u8, element: *const Element) bool {
-    return std.mem.eql(u8, normalized_id, "id") and element.asConstNode().isConnected();
+fn shouldAddToIdMap(normalized_name: []const u8, element: *Element) bool {
+    if (!std.mem.eql(u8, normalized_name, "id")) {
+        return false;
+    }
+
+    const node = element.asNode();
+    // Shadow tree elements are always added to their shadow root's map
+    if (node.isInShadowTree()) {
+        return true;
+    }
+    // Document tree elements only when connected
+    return node.isConnected();
 }
 
 pub fn normalizeNameForLookup(name: []const u8, page: *Page) ![]const u8 {
diff --git a/src/browser/webapi/element/html/Custom.zig b/src/browser/webapi/element/html/Custom.zig
index abf8a341..31e46bcf 100644
--- a/src/browser/webapi/element/html/Custom.zig
+++ b/src/browser/webapi/element/html/Custom.zig
@@ -132,7 +132,6 @@ pub fn checkAndAttachBuiltIn(element: *Element, page: *Page) !void {
     };
 }
 
-
 fn invokeCallback(self: *Custom, comptime callback_name: [:0]const u8, args: anytype, page: *Page) void {
     if (self._definition == null) {
         return;
diff --git a/src/browser/webapi/selector/List.zig b/src/browser/webapi/selector/List.zig
index 449fc70b..8f91d8ec 100644
--- a/src/browser/webapi/selector/List.zig
+++ b/src/browser/webapi/selector/List.zig
@@ -106,7 +106,7 @@ fn optimizeSelector(root: *Node, selector: *const Selector.Selector, page: *Page
     const segment_index = anchor.segment_index;
 
     // Look up the element by ID (O(1) hash map lookup)
-    const id_element = page.document._elements_by_id.get(id) orelse return null;
+    const id_element = page.getElementByIdFromNode(root, id) orelse return null;
     const id_node = id_element.asNode();
 
     if (!root.contains(id_node)) {
diff --git a/src/browser/webapi/selector/Selector.zig b/src/browser/webapi/selector/Selector.zig
index 3f72c442..bed15801 100644
--- a/src/browser/webapi/selector/Selector.zig
+++ b/src/browser/webapi/selector/Selector.zig
@@ -36,7 +36,7 @@ pub fn querySelector(root: *Node, input: []const u8, page: *Page) !?*Node.Elemen
         if (selector.segments.len == 0 and selector.first.parts.len == 1) {
             const first = selector.first.parts[0];
             if (first == .id) {
-                const el = page.document._elements_by_id.get(first.id) orelse continue;
+                const el = page.getElementByIdFromNode(root, first.id) orelse continue;
                 // Check if the element is within the root subtree
                 if (root.contains(el.asNode())) {
                     return el;