diff --git a/src/browser/Page.zig b/src/browser/Page.zig
index 28c1fcac..80500316 100644
--- a/src/browser/Page.zig
+++ b/src/browser/Page.zig
@@ -1024,6 +1024,12 @@ pub fn createElement(self: *Page, ns_: ?[]const u8, name: []const u8, attribute_
             else => {},
         },
         4 => switch (@as(u32, @bitCast(name[0..4].*))) {
+            asUint("span") => return self.createHtmlElementT(
+                Element.Html.Generic,
+                namespace,
+                attribute_iterator,
+                .{ ._proto = undefined, ._tag_name = String.init(undefined, "span", .{}) catch unreachable, ._tag = .span },
+            ),
             asUint("meta") => return self.createHtmlElementT(
                 Element.Html.Meta,
                 namespace,
@@ -1036,6 +1042,12 @@ pub fn createElement(self: *Page, ns_: ?[]const u8, name: []const u8, attribute_
                 attribute_iterator,
                 .{ ._proto = undefined },
             ),
+            asUint("slot") => return self.createHtmlElementT(
+                Element.Html.Slot,
+                namespace,
+                attribute_iterator,
+                .{ ._proto = undefined },
+            ),
             asUint("html") => return self.createHtmlElementT(
                 Element.Html.Html,
                 namespace,
@@ -1066,12 +1078,6 @@ pub fn createElement(self: *Page, ns_: ?[]const u8, name: []const u8, attribute_
                 attribute_iterator,
                 .{ ._proto = undefined, ._tag_name = String.init(undefined, "main", .{}) catch unreachable, ._tag = .main },
             ),
-            asUint("span") => return self.createHtmlElementT(
-                Element.Html.Generic,
-                namespace,
-                attribute_iterator,
-                .{ ._proto = undefined, ._tag_name = String.init(undefined, "span", .{}) catch unreachable, ._tag = .span },
-            ),
             else => {},
         },
         5 => switch (@as(u40, @bitCast(name[0..5].*))) {
@@ -1787,3 +1793,7 @@ const testing = @import("../testing.zig");
 test "WebApi: Page" {
     try testing.htmlRunner("page", .{});
 }
+
+test "WebApi: Integration" {
+    try testing.htmlRunner("integration", .{});
+}
diff --git a/src/browser/ScriptManager.zig b/src/browser/ScriptManager.zig
index 632be5f2..0d421db1 100644
--- a/src/browser/ScriptManager.zig
+++ b/src/browser/ScriptManager.zig
@@ -249,11 +249,14 @@ pub fn addFromElement(self: *ScriptManager, script_element: *Element.Html.Script
             .error_callback = Script.errorCallback,
         });
 
-        log.debug(.http, "script queue", .{
-            .ctx = ctx,
-            .url = remote_url.?,
-            .stack = page.js.stackTrace() catch "???",
-        });
+        if (comptime IS_DEBUG) {
+            log.debug(.http, "script queue", .{
+                .ctx = ctx,
+                .url = remote_url.?,
+                .element = element,
+                .stack = page.js.stackTrace() catch "???",
+            });
+        }
     }
 
     if (script.mode != .normal) {
@@ -326,12 +329,14 @@ pub fn preloadImport(self: *ScriptManager, url: [:0]const u8, referrer: []const
     var headers = try self.client.newHeaders();
     try self.page.requestCookie(.{}).headersForRequest(self.page.arena, url, &headers);
 
-    log.debug(.http, "script queue", .{
-        .url = url,
-        .ctx = "module",
-        .referrer = referrer,
-        .stack = self.page.js.stackTrace() catch "???",
-    });
+    if (comptime IS_DEBUG) {
+        log.debug(.http, "script queue", .{
+            .url = url,
+            .ctx = "module",
+            .referrer = referrer,
+            .stack = self.page.js.stackTrace() catch "???",
+        });
+    }
 
     try self.client.request(.{
         .url = url,
@@ -403,12 +408,14 @@ pub fn getAsyncImport(self: *ScriptManager, url: [:0]const u8, cb: ImportAsync.C
     var headers = try self.client.newHeaders();
     try self.page.requestCookie(.{}).headersForRequest(self.page.arena, url, &headers);
 
-    log.debug(.http, "script queue", .{
-        .url = url,
-        .ctx = "dynamic module",
-        .referrer = referrer,
-        .stack = self.page.js.stackTrace() catch "???",
-    });
+    if (comptime IS_DEBUG) {
+        log.debug(.http, "script queue", .{
+            .url = url,
+            .ctx = "dynamic module",
+            .referrer = referrer,
+            .stack = self.page.js.stackTrace() catch "???",
+        });
+    }
 
     // It's possible, but unlikely, for client.request to immediately finish
     // a request, thus calling our callback. We generally don't want a call
@@ -617,11 +624,13 @@ const Script = struct {
             return;
         }
 
-        log.debug(.http, "script header", .{
-            .req = transfer,
-            .status = header.status,
-            .content_type = header.contentType(),
-        });
+        if (comptime IS_DEBUG) {
+            log.debug(.http, "script header", .{
+                .req = transfer,
+                .status = header.status,
+                .content_type = header.contentType(),
+            });
+        }
 
         // If this isn't true, then we'll likely leak memory. If you don't
         // set `CURLOPT_SUPPRESS_CONNECT_HEADERS` and CONNECT to a proxy, this
@@ -649,7 +658,9 @@ const Script = struct {
     fn doneCallback(ctx: *anyopaque) !void {
         const self: *Script = @ptrCast(@alignCast(ctx));
         self.complete = true;
-        log.debug(.http, "script fetch complete", .{ .req = self.url });
+        if (comptime IS_DEBUG) {
+            log.debug(.http, "script fetch complete", .{ .req = self.url });
+        }
 
         const manager = self.manager;
         if (self.mode == .async or self.mode == .import_async) {
diff --git a/src/browser/js/bridge.zig b/src/browser/js/bridge.zig
index 7ac4f0ad..b93c3ec0 100644
--- a/src/browser/js/bridge.zig
+++ b/src/browser/js/bridge.zig
@@ -538,6 +538,7 @@ pub const JsApis = flattenTypes(&.{
     @import("../webapi/element/html/Paragraph.zig"),
     @import("../webapi/element/html/Script.zig"),
     @import("../webapi/element/html/Select.zig"),
+    @import("../webapi/element/html/Slot.zig"),
     @import("../webapi/element/html/Style.zig"),
     @import("../webapi/element/html/Template.zig"),
     @import("../webapi/element/html/TextArea.zig"),
@@ -574,4 +575,5 @@ pub const JsApis = flattenTypes(&.{
     @import("../webapi/Blob.zig"),
     @import("../webapi/File.zig"),
     @import("../webapi/Screen.zig"),
+    @import("../webapi/PerformanceObserver.zig"),
 });
diff --git a/src/browser/tests/element/html/slot.html b/src/browser/tests/element/html/slot.html
new file mode 100644
index 00000000..af2b0808
--- /dev/null
+++ b/src/browser/tests/element/html/slot.html
@@ -0,0 +1,384 @@
+<!DOCTYPE html>
+<script src="../../testing.js"></script>
+
+<script id="Slot#basic_creation">
+{
+    const slot = document.createElement('slot');
+    testing.expectEqual('SLOT', slot.tagName);
+}
+</script>
+
+<script id="Slot#name_attribute">
+{
+    const slot = document.createElement('slot');
+
+    // Set name via setAttribute
+    slot.setAttribute('name', 'header');
+    testing.expectEqual('header', slot.getAttribute('name'));
+
+    // Change name
+    slot.setAttribute('name', 'footer');
+    testing.expectEqual('footer', slot.getAttribute('name'));
+}
+</script>
+
+<script id="Slot#assignedNodes_empty">
+{
+    // Slot not in shadow tree
+    const slot = document.createElement('slot');
+    const nodes = slot.assignedNodes();
+    testing.expectEqual(0, nodes.length);
+}
+</script>
+
+<script id="Slot#assignedElements_empty">
+{
+    // Slot not in shadow tree
+    const slot = document.createElement('slot');
+    const elements = slot.assignedElements();
+    testing.expectEqual(0, elements.length);
+}
+</script>
+
+<script id="Slot#default_slot_basic">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+
+    // Create default slot (no name)
+    const slot = document.createElement('slot');
+    shadow.appendChild(slot);
+
+    // Add content to host
+    const span1 = document.createElement('span');
+    span1.textContent = 'Content 1';
+    host.appendChild(span1);
+
+    const span2 = document.createElement('span');
+    span2.textContent = 'Content 2';
+    host.appendChild(span2);
+
+    // Both spans should be assigned to default slot
+    const nodes = slot.assignedNodes();
+    testing.expectEqual(2, nodes.length);
+    testing.expectTrue(nodes[0] === span1);
+    testing.expectTrue(nodes[1] === span2);
+}
+</script>
+
+<script id="Slot#named_slot">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+
+    // Create named slot
+    const headerSlot = document.createElement('slot');
+    headerSlot.name = 'header';
+    shadow.appendChild(headerSlot);
+
+    // Add content with slot attribute
+    const h1 = document.createElement('h1');
+    h1.textContent = 'Title';
+    h1.setAttribute('slot', 'header');
+    host.appendChild(h1);
+
+    const p = document.createElement('p');
+    p.textContent = 'Body';
+    host.appendChild(p);
+
+    // Only h1 should be assigned to header slot
+    const nodes = headerSlot.assignedNodes();
+    testing.expectEqual(1, nodes.length);
+    testing.expectTrue(nodes[0] === h1);
+}
+</script>
+
+<script id="Slot#multiple_slots">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+
+    // Create multiple named slots
+    const headerSlot = document.createElement('slot');
+    headerSlot.name = 'header';
+    shadow.appendChild(headerSlot);
+
+    const footerSlot = document.createElement('slot');
+    footerSlot.name = 'footer';
+    shadow.appendChild(footerSlot);
+
+    const defaultSlot = document.createElement('slot');
+    shadow.appendChild(defaultSlot);
+
+    // Add content
+    const h1 = document.createElement('h1');
+    h1.setAttribute('slot', 'header');
+    host.appendChild(h1);
+
+    const p = document.createElement('p');
+    host.appendChild(p);
+
+    const footer = document.createElement('footer');
+    footer.setAttribute('slot', 'footer');
+    host.appendChild(footer);
+
+    // Check each slot
+    const headerNodes = headerSlot.assignedNodes();
+    testing.expectEqual(1, headerNodes.length);
+    testing.expectTrue(headerNodes[0] === h1);
+
+    const footerNodes = footerSlot.assignedNodes();
+    testing.expectEqual(1, footerNodes.length);
+    testing.expectTrue(footerNodes[0] === footer);
+
+    const defaultNodes = defaultSlot.assignedNodes();
+    testing.expectEqual(1, defaultNodes.length);
+    testing.expectTrue(defaultNodes[0] === p);
+}
+</script>
+
+<script id="Slot#assignedElements_filters_text_nodes">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+
+    const slot = document.createElement('slot');
+    shadow.appendChild(slot);
+
+    // Add mixed content
+    const span = document.createElement('span');
+    host.appendChild(span);
+
+    const text = document.createTextNode('Some text');
+    host.appendChild(text);
+
+    const div = document.createElement('div');
+    host.appendChild(div);
+
+    // assignedNodes should include all
+    const nodes = slot.assignedNodes();
+    testing.expectEqual(3, nodes.length);
+
+    // assignedElements should only include elements
+    const elements = slot.assignedElements();
+    testing.expectEqual(2, elements.length);
+    testing.expectTrue(elements[0] === span);
+    testing.expectTrue(elements[1] === div);
+}
+</script>
+
+<script id="Slot#text_nodes_default_slot_only">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+
+    const namedSlot = document.createElement('slot');
+    namedSlot.name = 'named';
+    shadow.appendChild(namedSlot);
+
+    const defaultSlot = document.createElement('slot');
+    shadow.appendChild(defaultSlot);
+
+    // Add text node
+    const text = document.createTextNode('Text content');
+    host.appendChild(text);
+
+    // Text should go to default slot only
+    const namedNodes = namedSlot.assignedNodes();
+    testing.expectEqual(0, namedNodes.length);
+
+    const defaultNodes = defaultSlot.assignedNodes();
+    testing.expectEqual(1, defaultNodes.length);
+    testing.expectTrue(defaultNodes[0] === text);
+}
+</script>
+
+<script id="Slot#flatten_false">
+{
+    const outerHost = document.createElement('div');
+    const outerShadow = outerHost.attachShadow({ mode: 'open' });
+
+    const innerHost = document.createElement('div');
+    const innerShadow = innerHost.attachShadow({ mode: 'open' });
+
+    // Inner slot
+    const innerSlot = document.createElement('slot');
+    innerShadow.appendChild(innerSlot);
+
+    // Outer slot contains inner host
+    const outerSlot = document.createElement('slot');
+    outerShadow.appendChild(outerSlot);
+    outerHost.appendChild(innerHost);
+
+    // Add content to inner host
+    const span = document.createElement('span');
+    innerHost.appendChild(span);
+
+    // Without flatten, outer slot should see inner host (not span)
+    const nodes = outerSlot.assignedNodes();
+    testing.expectEqual(1, nodes.length);
+    testing.expectTrue(nodes[0] === innerHost);
+}
+</script>
+
+<script id="Slot#flatten_true">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+
+    const slot = document.createElement('slot');
+    shadow.appendChild(slot);
+
+    // Add an element with a nested slot in it
+    const container = document.createElement('div');
+    container.innerHTML = '<slot name="nested"></slot>';
+    host.appendChild(container);
+
+    // Add a regular span
+    const span = document.createElement('span');
+    host.appendChild(span);
+
+    // Without flatten: should see container and span (2 elements)
+    const nodesNoFlat = slot.assignedNodes({ flatten: false });
+    testing.expectEqual(2, nodesNoFlat.length);
+
+    // With flatten: should still see container and span
+    // (flatten only matters if the assigned node is itself a slot in a shadow tree)
+    const nodesFlat = slot.assignedNodes({ flatten: true });
+    testing.expectEqual(2, nodesFlat.length);
+}
+</script>
+
+<script id="Slot#assignedElements_with_flatten">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+
+    const slot = document.createElement('slot');
+    shadow.appendChild(slot);
+
+    // Add mixed content
+    const div = document.createElement('div');
+    host.appendChild(div);
+
+    const text = document.createTextNode('text');
+    host.appendChild(text);
+
+    const span = document.createElement('span');
+    host.appendChild(span);
+
+    // assignedElements with flatten should only return elements
+    const elements = slot.assignedElements({ flatten: true });
+    testing.expectEqual(2, elements.length);
+    testing.expectTrue(elements[0] === div);
+    testing.expectTrue(elements[1] === span);
+}
+</script>
+
+<script id="Slot#empty_slot_name_matches_no_slot_attribute">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+
+    const defaultSlot = document.createElement('slot');
+    shadow.appendChild(defaultSlot);
+
+    // Element without slot attribute
+    const div = document.createElement('div');
+    host.appendChild(div);
+
+    // Element with empty slot attribute
+    const span = document.createElement('span');
+    span.setAttribute('slot', '');
+    host.appendChild(span);
+
+    // Both should go to default slot
+    const nodes = defaultSlot.assignedNodes();
+    testing.expectEqual(2, nodes.length);
+    testing.expectTrue(nodes[0] === div);
+    testing.expectTrue(nodes[1] === span);
+}
+</script>
+
+<script id="Slot#slot_attribute_case_sensitive">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+
+    const slot = document.createElement('slot');
+    slot.name = 'MySlot';
+    shadow.appendChild(slot);
+
+    // Matching case
+    const div1 = document.createElement('div');
+    div1.setAttribute('slot', 'MySlot');
+    host.appendChild(div1);
+
+    // Different case
+    const div2 = document.createElement('div');
+    div2.setAttribute('slot', 'myslot');
+    host.appendChild(div2);
+
+    // Only exact match should be assigned
+    const nodes = slot.assignedNodes();
+    testing.expectEqual(1, nodes.length);
+    testing.expectTrue(nodes[0] === div1);
+}
+</script>
+
+<script id="Slot#slot_in_slot_shadow_tree">
+{
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+
+    // Create a container in shadow tree
+    const container = document.createElement('div');
+    shadow.appendChild(container);
+
+    // Slot inside container
+    const slot = document.createElement('slot');
+    container.appendChild(slot);
+
+    // Add content to host
+    const span = document.createElement('span');
+    host.appendChild(span);
+
+    // Slot should still work even when not direct child of shadow root
+    const nodes = slot.assignedNodes();
+    testing.expectEqual(1, nodes.length);
+    testing.expectTrue(nodes[0] === span);
+}
+</script>
+
+<script id="Slot#flatten_with_slot_and_siblings">
+{
+    // Test that flatten continues processing siblings after a nested slot
+    const host = document.createElement('div');
+    const shadow = host.attachShadow({ mode: 'open' });
+
+    const outerSlot = document.createElement('slot');
+    shadow.appendChild(outerSlot);
+
+    // Add a nested slot as first child
+    const innerSlot = document.createElement('slot');
+    innerSlot.setAttribute('name', 'inner');
+    host.appendChild(innerSlot);
+
+    // Add a regular element after the slot
+    const div = document.createElement('div');
+    div.textContent = 'After slot';
+    host.appendChild(div);
+
+    // Add another element
+    const span = document.createElement('span');
+    span.textContent = 'Another element';
+    host.appendChild(span);
+
+    // With flatten=true, should get all elements including those after the nested slot
+    const flattened = outerSlot.assignedElements({ flatten: true });
+    testing.expectEqual(3, flattened.length);
+    testing.expectTrue(flattened[0] === innerSlot);
+    testing.expectTrue(flattened[1] === div);
+    testing.expectTrue(flattened[2] === span);
+}
+</script>
diff --git a/src/browser/tests/integration/custom_element_composition.html b/src/browser/tests/integration/custom_element_composition.html
new file mode 100644
index 00000000..3559d9f8
--- /dev/null
+++ b/src/browser/tests/integration/custom_element_composition.html
@@ -0,0 +1,456 @@
+<!DOCTYPE html>
+<body></body>
+<script src="../testing.js"></script>
+
+<!-- Test complex component composition patterns like React/Next.js -->
+
+<script id="card_component_with_slots">
+{
+    // Define a card component with header, body, footer slots (like React children props)
+    customElements.define('app-card', class extends HTMLElement {
+        constructor() {
+            super();
+            this.attachShadow({ mode: 'open' });
+            this.shadowRoot.innerHTML = `
+                <div class="card">
+                    <div class="card-header">
+                        <slot name="header">Default Header</slot>
+                    </div>
+                    <div class="card-body">
+                        <slot></slot>
+                    </div>
+                    <div class="card-footer">
+                        <slot name="footer"></slot>
+                    </div>
+                </div>
+            `;
+        }
+    });
+
+    // Create card with mixed content
+    const card = document.createElement('app-card');
+
+    const header = document.createElement('h2');
+    header.setAttribute('slot', 'header');
+    header.textContent = 'My Card';
+    card.appendChild(header);
+
+    const body = document.createElement('p');
+    body.textContent = 'Card content';
+    card.appendChild(body);
+
+    const footer = document.createElement('span');
+    footer.setAttribute('slot', 'footer');
+    footer.textContent = 'Footer text';
+    card.appendChild(footer);
+
+    document.body.appendChild(card);
+
+    // Verify slot assignments
+    const headerSlot = card.shadowRoot.querySelector('slot[name="header"]');
+    const defaultSlot = card.shadowRoot.querySelectorAll('slot:not([name])')[0];
+    const footerSlot = card.shadowRoot.querySelector('slot[name="footer"]');
+
+    testing.expectEqual(1, headerSlot.assignedElements().length);
+    testing.expectEqual(1, defaultSlot.assignedElements().length);
+    testing.expectEqual(1, footerSlot.assignedElements().length);
+
+    testing.expectTrue(headerSlot.assignedElements()[0] === header);
+    testing.expectTrue(defaultSlot.assignedElements()[0] === body);
+    testing.expectTrue(footerSlot.assignedElements()[0] === footer);
+}
+</script>
+
+<script id="nested_components_with_slots">
+{
+    // Like React HOCs or wrapper components
+    customElements.define('app-wrapper', class extends HTMLElement {
+        constructor() {
+            super();
+            this.attachShadow({ mode: 'open' });
+            this.shadowRoot.innerHTML = `
+                <div class="wrapper">
+                    <slot></slot>
+                </div>
+            `;
+        }
+    });
+
+    customElements.define('app-inner', class extends HTMLElement {
+        constructor() {
+            super();
+            this.attachShadow({ mode: 'open' });
+            this.shadowRoot.innerHTML = `
+                <div class="inner">
+                    <slot></slot>
+                </div>
+            `;
+        }
+    });
+
+    // Nest components
+    const wrapper = document.createElement('app-wrapper');
+    const inner = document.createElement('app-inner');
+    const content = document.createElement('span');
+    content.textContent = 'Deeply nested';
+
+    inner.appendChild(content);
+    wrapper.appendChild(inner);
+    document.body.appendChild(wrapper);
+
+    // Verify outer wrapper slot
+    const wrapperSlot = wrapper.shadowRoot.querySelector('slot');
+    const wrapperAssigned = wrapperSlot.assignedElements();
+    testing.expectEqual(1, wrapperAssigned.length);
+    testing.expectTrue(wrapperAssigned[0] === inner);
+
+    // Verify inner component slot
+    const innerSlot = inner.shadowRoot.querySelector('slot');
+    const innerAssigned = innerSlot.assignedElements();
+    testing.expectEqual(1, innerAssigned.length);
+    testing.expectTrue(innerAssigned[0] === content);
+
+    // With flatten, outer slot should see through to content
+    const flatAssigned = wrapperSlot.assignedElements({ flatten: true });
+    testing.expectEqual(1, flatAssigned.length);
+}
+</script>
+
+<script id="tab_component_pattern">
+{
+    // Tabs pattern - common in UI libraries
+    customElements.define('app-tabs', class extends HTMLElement {
+        constructor() {
+            super();
+            this.attachShadow({ mode: 'open' });
+            this.shadowRoot.innerHTML = `
+                <div class="tabs">
+                    <div class="tab-buttons">
+                        <slot name="tabs"></slot>
+                    </div>
+                    <div class="tab-content">
+                        <slot name="panels"></slot>
+                    </div>
+                </div>
+            `;
+        }
+    });
+
+    const tabs = document.createElement('app-tabs');
+
+    // Add tab buttons
+    for (let i = 1; i <= 3; i++) {
+        const button = document.createElement('button');
+        button.setAttribute('slot', 'tabs');
+        button.textContent = `Tab ${i}`;
+        tabs.appendChild(button);
+    }
+
+    // Add panels
+    for (let i = 1; i <= 3; i++) {
+        const panel = document.createElement('div');
+        panel.setAttribute('slot', 'panels');
+        panel.textContent = `Panel ${i}`;
+        tabs.appendChild(panel);
+    }
+
+    document.body.appendChild(tabs);
+
+    const tabSlot = tabs.shadowRoot.querySelector('slot[name="tabs"]');
+    const panelSlot = tabs.shadowRoot.querySelector('slot[name="panels"]');
+
+    testing.expectEqual(3, tabSlot.assignedElements().length);
+    testing.expectEqual(3, panelSlot.assignedElements().length);
+}
+</script>
+
+<script id="layout_component_all_slots">
+{
+    // App layout pattern from Next.js/React
+    customElements.define('app-layout', class extends HTMLElement {
+        constructor() {
+            super();
+            this.attachShadow({ mode: 'open' });
+            this.shadowRoot.innerHTML = `
+                <div class="layout">
+                    <header><slot name="header"></slot></header>
+                    <aside><slot name="sidebar"></slot></aside>
+                    <main><slot></slot></main>
+                    <footer><slot name="footer"></slot></footer>
+                </div>
+            `;
+        }
+    });
+
+    const layout = document.createElement('app-layout');
+
+    const header = document.createElement('div');
+    header.setAttribute('slot', 'header');
+    header.textContent = 'Header';
+    layout.appendChild(header);
+
+    const sidebar = document.createElement('nav');
+    sidebar.setAttribute('slot', 'sidebar');
+    sidebar.textContent = 'Nav';
+    layout.appendChild(sidebar);
+
+    const main = document.createElement('article');
+    main.textContent = 'Main content';
+    layout.appendChild(main);
+
+    const footer = document.createElement('div');
+    footer.setAttribute('slot', 'footer');
+    footer.textContent = 'Footer';
+    layout.appendChild(footer);
+
+    document.body.appendChild(layout);
+
+    // All slots should be filled
+    const slots = layout.shadowRoot.querySelectorAll('slot');
+    testing.expectEqual(4, slots.length);
+
+    for (const slot of slots) {
+        testing.expectTrue(slot.assignedNodes().length > 0);
+    }
+}
+</script>
+
+<script id="dynamic_slot_reassignment">
+{
+    // Simulating conditional rendering like React
+    customElements.define('app-container', class extends HTMLElement {
+        constructor() {
+            super();
+            this.attachShadow({ mode: 'open' });
+            this.shadowRoot.innerHTML = `
+                <slot name="primary"></slot>
+                <slot name="secondary"></slot>
+            `;
+        }
+    });
+
+    const container = document.createElement('app-container');
+    const elem = document.createElement('div');
+    elem.textContent = 'Content';
+    elem.setAttribute('slot', 'primary');
+    container.appendChild(elem);
+    document.body.appendChild(container);
+
+    const primarySlot = container.shadowRoot.querySelector('slot[name="primary"]');
+    const secondarySlot = container.shadowRoot.querySelector('slot[name="secondary"]');
+
+    // Initially in primary
+    testing.expectEqual(1, primarySlot.assignedElements().length);
+    testing.expectEqual(0, secondarySlot.assignedElements().length);
+
+    // Move to secondary (like React re-rendering with different props)
+    elem.setAttribute('slot', 'secondary');
+
+    // Should now be in secondary
+    testing.expectEqual(0, primarySlot.assignedElements().length);
+    testing.expectEqual(1, secondarySlot.assignedElements().length);
+}
+</script>
+
+<script id="list_rendering_pattern">
+{
+    // List rendering like map() in React
+    customElements.define('app-list', class extends HTMLElement {
+        constructor() {
+            super();
+            this.attachShadow({ mode: 'open' });
+            this.shadowRoot.innerHTML = `
+                <ul><slot></slot></ul>
+            `;
+        }
+    });
+
+    customElements.define('app-list-item', class extends HTMLElement {
+        constructor() {
+            super();
+            this.attachShadow({ mode: 'open' });
+            this.shadowRoot.innerHTML = `
+                <li><slot></slot></li>
+            `;
+        }
+    });
+
+    const list = document.createElement('app-list');
+
+    const items = ['Item 1', 'Item 2', 'Item 3'];
+    items.forEach(text => {
+        const item = document.createElement('app-list-item');
+        const span = document.createElement('span');
+        span.textContent = text;
+        item.appendChild(span);
+        list.appendChild(item);
+    });
+
+    document.body.appendChild(list);
+
+    // List should have 3 items assigned
+    const listSlot = list.shadowRoot.querySelector('slot');
+    testing.expectEqual(3, listSlot.assignedElements().length);
+
+    // Each item should have content
+    const itemElements = listSlot.assignedElements();
+    itemElements.forEach(item => {
+        const itemSlot = item.shadowRoot.querySelector('slot');
+        testing.expectEqual(1, itemSlot.assignedElements().length);
+    });
+}
+</script>
+
+<script id="fallback_content_pattern">
+{
+    // Default/fallback content like React default props
+    customElements.define('app-message', class extends HTMLElement {
+        constructor() {
+            super();
+            this.attachShadow({ mode: 'open' });
+            this.shadowRoot.innerHTML = `
+                <div class="message">
+                    <slot>No message provided</slot>
+                </div>
+            `;
+        }
+    });
+
+    // Without content - should use fallback
+    const empty = document.createElement('app-message');
+    document.body.appendChild(empty);
+    const emptySlot = empty.shadowRoot.querySelector('slot');
+    testing.expectEqual(0, emptySlot.assignedNodes().length);
+
+    // With content - should override fallback
+    const filled = document.createElement('app-message');
+    const text = document.createTextNode('Custom message');
+    filled.appendChild(text);
+    document.body.appendChild(filled);
+    const filledSlot = filled.shadowRoot.querySelector('slot');
+    testing.expectEqual(1, filledSlot.assignedNodes().length);
+}
+</script>
+
+<script id="form_custom_input">
+{
+    // Custom form input component
+    customElements.define('app-input', class extends HTMLElement {
+        constructor() {
+            super();
+            this.attachShadow({ mode: 'open' });
+            this.shadowRoot.innerHTML = `
+                <label>
+                    <slot name="label">Label</slot>
+                    <input type="text">
+                </label>
+            `;
+        }
+    });
+
+    const input = document.createElement('app-input');
+    const label = document.createElement('span');
+    label.setAttribute('slot', 'label');
+    label.textContent = 'Username:';
+    input.appendChild(label);
+
+    document.body.appendChild(input);
+
+    const labelSlot = input.shadowRoot.querySelector('slot[name="label"]');
+    testing.expectEqual(1, labelSlot.assignedElements().length);
+    testing.expectEqual('Username:', labelSlot.assignedElements()[0].textContent);
+}
+</script>
+
+<script id="deeply_nested_slot_chain">
+{
+    // Three levels of nesting with slots
+    customElements.define('app-outer', class extends HTMLElement {
+        constructor() {
+            super();
+            this.attachShadow({ mode: 'open' });
+            this.shadowRoot.innerHTML = `<div class="outer"><slot></slot></div>`;
+        }
+    });
+
+    customElements.define('app-middle', class extends HTMLElement {
+        constructor() {
+            super();
+            this.attachShadow({ mode: 'open' });
+            this.shadowRoot.innerHTML = `<div class="middle"><slot></slot></div>`;
+        }
+    });
+
+    customElements.define('app-leaf', class extends HTMLElement {
+        constructor() {
+            super();
+            this.attachShadow({ mode: 'open' });
+            this.shadowRoot.innerHTML = `<div class="leaf"><slot></slot></div>`;
+        }
+    });
+
+    const outer = document.createElement('app-outer');
+    const middle = document.createElement('app-middle');
+    const leaf = document.createElement('app-leaf');
+    const content = document.createElement('span');
+    content.textContent = 'Deep content';
+
+    leaf.appendChild(content);
+    middle.appendChild(leaf);
+    outer.appendChild(middle);
+    document.body.appendChild(outer);
+
+    // Each level should see one element
+    const outerSlot = outer.shadowRoot.querySelector('slot');
+    testing.expectEqual(1, outerSlot.assignedElements().length);
+    testing.expectTrue(outerSlot.assignedElements()[0] === middle);
+
+    const middleSlot = middle.shadowRoot.querySelector('slot');
+    testing.expectEqual(1, middleSlot.assignedElements().length);
+    testing.expectTrue(middleSlot.assignedElements()[0] === leaf);
+
+    const leafSlot = leaf.shadowRoot.querySelector('slot');
+    testing.expectEqual(1, leafSlot.assignedElements().length);
+    testing.expectTrue(leafSlot.assignedElements()[0] === content);
+
+    // Flatten should not traverse through non-slot elements
+    const outerFlat = outerSlot.assignedElements({ flatten: true });
+    testing.expectEqual(1, outerFlat.length);
+}
+</script>
+
+<script id="mixed_slotted_and_unslotted">
+{
+    // Some children slotted, some not (edge case)
+    customElements.define('app-mixed', class extends HTMLElement {
+        constructor() {
+            super();
+            this.attachShadow({ mode: 'open' });
+            this.shadowRoot.innerHTML = `
+                <slot name="named"></slot>
+                <slot></slot>
+            `;
+        }
+    });
+
+    const mixed = document.createElement('app-mixed');
+
+    const named = document.createElement('div');
+    named.setAttribute('slot', 'named');
+    mixed.appendChild(named);
+
+    const unnamed1 = document.createElement('span');
+    mixed.appendChild(unnamed1);
+
+    const unnamed2 = document.createElement('p');
+    mixed.appendChild(unnamed2);
+
+    document.body.appendChild(mixed);
+
+    const namedSlot = mixed.shadowRoot.querySelector('slot[name="named"]');
+    const defaultSlot = mixed.shadowRoot.querySelectorAll('slot:not([name])')[0];
+
+    testing.expectEqual(1, namedSlot.assignedElements().length);
+    testing.expectEqual(2, defaultSlot.assignedElements().length);
+}
+</script>
diff --git a/src/browser/tests/window/window.html b/src/browser/tests/window/window.html
index c378c130..9cd74b37 100644
--- a/src/browser/tests/window/window.html
+++ b/src/browser/tests/window/window.html
@@ -102,4 +102,3 @@
   testing.expectEqual(24, screen.pixelDepth);
   testing.expectEqual(screen, window.screen);
 </script>
-
diff --git a/src/browser/webapi/Element.zig b/src/browser/webapi/Element.zig
index 023da510..fb9b927e 100644
--- a/src/browser/webapi/Element.zig
+++ b/src/browser/webapi/Element.zig
@@ -148,6 +148,7 @@ pub fn getTagNameLower(self: *const Element) []const u8 {
                 .p => "p",
                 .script => "script",
                 .select => "select",
+                .slot => "slot",
                 .style => "style",
                 .template => "template",
                 .text_area => "textarea",
@@ -192,6 +193,7 @@ pub fn getTagNameSpec(self: *const Element, buf: []u8) []const u8 {
                 .p => "P",
                 .script => "SCRIPT",
                 .select => "SELECT",
+                .slot => "SLOT",
                 .style => "STYLE",
                 .template => "TEMPLATE",
                 .text_area => "TEXTAREA",
@@ -790,6 +792,7 @@ pub fn getTag(self: *const Element) Tag {
             .generic => |g| g._tag,
             .script => .script,
             .select => .select,
+            .slot => .slot,
             .option => .option,
             .template => .template,
             .text_area => .textarea,
@@ -855,6 +858,7 @@ pub const Tag = enum {
     rect,
     script,
     select,
+    slot,
     span,
     strong,
     style,
diff --git a/src/browser/webapi/Performance.zig b/src/browser/webapi/Performance.zig
index 3ac87871..60b972a3 100644
--- a/src/browser/webapi/Performance.zig
+++ b/src/browser/webapi/Performance.zig
@@ -1,6 +1,13 @@
 const js = @import("../js/js.zig");
 const datetime = @import("../../datetime.zig");
 
+pub fn registerTypes() []const type {
+    return &.{
+        Performance,
+        Entry,
+    };
+}
+
 const Performance = @This();
 
 _time_origin: u64,
@@ -34,6 +41,65 @@ pub const JsApi = struct {
     pub const timeOrigin = bridge.accessor(Performance.getTimeOrigin, null, .{});
 };
 
+pub const Entry = struct {
+    _duration: f64 = 0.0,
+    _entry_type: Type,
+    _name: []const u8,
+    _start_time: f64 = 0.0,
+
+    const Type = enum {
+        element,
+        event,
+        first_input,
+        largest_contentful_paint,
+        layout_shift,
+        long_animation_frame,
+        longtask,
+        mark,
+        measure,
+        navigation,
+        paint,
+        resource,
+        taskattribution,
+        visibility_state,
+    };
+
+    pub fn getDuration(self: *const Entry) f64 {
+        return self._duration;
+    }
+
+    pub fn getEntryType(self: *const Entry) []const u8 {
+       return switch (self._entry_type) {
+            .first_input => "first-input",
+            .largest_contentful_paint => "largest-contentful-paint",
+            .layout_shift => "layout-shift",
+            .long_animation_frame => "long-animation-frame",
+            .visibility_state => "visibility-state",
+            else => |t| @tagName(t),
+        };
+    }
+
+    pub fn getName(self: *const Entry) []const u8 {
+        return self._name;
+    }
+
+    pub fn getStartTime(self: *const Entry) f64 {
+        return self._start_time;
+    }
+
+    pub const JsApi = struct {
+        pub const bridge = js.Bridge(Entry);
+
+        pub const Meta = struct {
+            pub const name = "PerformanceEntry";
+            pub const prototype_chain = bridge.prototypeChain();
+            pub var class_id: bridge.ClassId = undefined;
+        };
+        pub const duration = bridge.accessor(Entry.getDuration, null, .{});
+        pub const entryType = bridge.accessor(Entry.getEntryType, null, .{});
+    };
+};
+
 const testing = @import("../../testing.zig");
 test "WebApi: Performance" {
     try testing.htmlRunner("performance.html", .{});
diff --git a/src/browser/webapi/PerformanceObserver.zig b/src/browser/webapi/PerformanceObserver.zig
new file mode 100644
index 00000000..08bc2733
--- /dev/null
+++ b/src/browser/webapi/PerformanceObserver.zig
@@ -0,0 +1,67 @@
+// 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 js = @import("../js/js.zig");
+
+const Entry = @import("Performance.zig").Entry;
+
+// https://developer.mozilla.org/en-US/docs/Web/API/PerformanceObserver
+const PerformanceObserver = @This();
+
+pub fn init(callback: js.Function) PerformanceObserver {
+    _ = callback;
+    return .{};
+}
+
+const ObserverOptions = struct {
+    buffered: ?bool = null,
+    durationThreshold: ?f64 = null,
+    entryTypes: ?[]const []const u8 = null,
+    type: ?[]const u8 = null,
+};
+
+pub fn observe(self: *const PerformanceObserver, opts_: ?ObserverOptions) void {
+    _ = self;
+    _ = opts_;
+    return;
+}
+
+pub fn disconnect(self: *PerformanceObserver) void {
+    _ = self;
+}
+
+pub fn takeRecords(_: *const PerformanceObserver) []const Entry {
+    return &.{};
+}
+
+pub const JsApi = struct {
+    pub const bridge = js.Bridge(PerformanceObserver);
+
+    pub const Meta = struct {
+        pub const name = "PerformanceObserver";
+        pub const prototype_chain = bridge.prototypeChain();
+        pub var class_id: bridge.ClassId = undefined;
+        pub const empty_with_no_proto = true;
+    };
+
+    pub const constructor = bridge.constructor(PerformanceObserver.init, .{});
+
+    pub const observe = bridge.function(PerformanceObserver.observe, .{});
+    pub const disconnect = bridge.function(PerformanceObserver.disconnect, .{});
+    pub const takeRecords = bridge.function(PerformanceObserver.takeRecords, .{});
+};
diff --git a/src/browser/webapi/element/Html.zig b/src/browser/webapi/element/Html.zig
index 475c1f21..4468b553 100644
--- a/src/browser/webapi/element/Html.zig
+++ b/src/browser/webapi/element/Html.zig
@@ -51,6 +51,7 @@ pub const Template = @import("html/Template.zig");
 pub const TextArea = @import("html/TextArea.zig");
 pub const Paragraph = @import("html/Paragraph.zig");
 pub const Select = @import("html/Select.zig");
+pub const Slot = @import("html/Slot.zig");
 pub const Option = @import("html/Option.zig");
 pub const IFrame = @import("html/IFrame.zig");
 
@@ -90,6 +91,7 @@ pub const Type = union(enum) {
     p: Paragraph,
     script: *Script,
     select: Select,
+    slot: Slot,
     style: Style,
     template: *Template,
     text_area: *TextArea,
@@ -131,6 +133,7 @@ pub fn className(self: *const HtmlElement) []const u8 {
         .generic => "[object HTMLElement]",
         .script => "[object HtmlScriptElement]",
         .select => "[object HTMLSelectElement]",
+        .slot => "[object HTMLSlotElement]",
         .template => "[object HTMLTemplateElement]",
         .option => "[object HTMLOptionElement]",
         .text_area => "[object HtmlTextAreaElement]",
diff --git a/src/browser/webapi/element/html/Script.zig b/src/browser/webapi/element/html/Script.zig
index c12038a6..e1f55988 100644
--- a/src/browser/webapi/element/html/Script.zig
+++ b/src/browser/webapi/element/html/Script.zig
@@ -57,6 +57,14 @@ pub fn setSrc(self: *Script, src: []const u8, page: *Page) !void {
     }
 }
 
+pub fn getType(self: *const Script) []const u8 {
+    return self.asConstElement().getAttributeSafe("type") orelse "";
+}
+
+pub fn setType(self: *Script, value: []const u8, page: *Page) !void {
+    return self.asElement().setAttributeSafe("type", value, page);
+}
+
 pub fn getOnLoad(self: *const Script) ?js.Function {
     return self._on_load;
 }
@@ -95,6 +103,7 @@ pub const JsApi = struct {
     };
 
     pub const src = bridge.accessor(Script.getSrc, Script.setSrc, .{});
+    pub const @"type" = bridge.accessor(Script.getType, Script.setType, .{});
     pub const onload = bridge.accessor(Script.getOnLoad, Script.setOnLoad, .{});
     pub const onerorr = bridge.accessor(Script.getOnError, Script.setOnError, .{});
     pub const noModule = bridge.accessor(Script.getNoModule, null, .{});
diff --git a/src/browser/webapi/element/html/Slot.zig b/src/browser/webapi/element/html/Slot.zig
new file mode 100644
index 00000000..1089ad56
--- /dev/null
+++ b/src/browser/webapi/element/html/Slot.zig
@@ -0,0 +1,151 @@
+const std = @import("std");
+
+const log = @import("../../../../log.zig");
+const js = @import("../../../js/js.zig");
+const Page = @import("../../../Page.zig");
+const Node = @import("../../Node.zig");
+const Element = @import("../../Element.zig");
+const HtmlElement = @import("../Html.zig");
+const ShadowRoot = @import("../../ShadowRoot.zig");
+
+const Slot = @This();
+
+_proto: *HtmlElement,
+
+pub fn asElement(self: *Slot) *Element {
+    return self._proto._proto;
+}
+
+pub fn asConstElement(self: *const Slot) *const Element {
+    return self._proto._proto;
+}
+
+pub fn asNode(self: *Slot) *Node {
+    return self.asElement().asNode();
+}
+
+pub fn getName(self: *const Slot) []const u8 {
+    return self.asConstElement().getAttributeSafe("name") orelse "";
+}
+
+pub fn setName(self: *Slot, name: []const u8, page: *Page) !void {
+    try self.asElement().setAttributeSafe("name", name, page);
+}
+
+const AssignedNodesOptions = struct {
+    flatten: bool = false,
+};
+
+pub fn assignedNodes(self: *Slot, opts_: ?AssignedNodesOptions, page: *Page) ![]const *Node {
+    const opts = opts_ orelse AssignedNodesOptions{};
+    var nodes: std.ArrayList(*Node) = .empty;
+    try self.collectAssignedNodes(false, &nodes, opts, page);
+    return nodes.items;
+}
+
+pub fn assignedElements(self: *Slot, opts_: ?AssignedNodesOptions, page: *Page) ![]const *Element {
+    const opts = opts_ orelse AssignedNodesOptions{};
+    var elements: std.ArrayList(*Element) = .empty;
+    try self.collectAssignedNodes(true, &elements, opts, page);
+    return elements.items;
+}
+
+fn CollectionType(comptime elements: bool) type {
+    return if (elements) *std.ArrayList(*Element) else *std.ArrayList(*Node);
+}
+
+fn collectAssignedNodes(self: *Slot, comptime elements: bool, coll: CollectionType(elements), opts: AssignedNodesOptions, page: *Page) !void {
+    // Find the shadow root this slot belongs to
+    const shadow_root = self.findShadowRoot() orelse return;
+
+    const slot_name = self.getName();
+    const allocator = page.call_arena;
+
+    const host = shadow_root.getHost();
+    var it = host.asNode().childrenIterator();
+    while (it.next()) |child| {
+        if (!isAssignedToSlot(child, slot_name)) {
+            continue;
+        }
+
+        if (opts.flatten) {
+            if (child.is(Slot)) |child_slot| {
+                // Only flatten if the child slot is actually in a shadow tree
+                if (child_slot.findShadowRoot()) |_| {
+                    try child_slot.collectAssignedNodes(elements, coll, opts, page);
+                    continue;
+                }
+                // Otherwise, treat it as a regular element and fall through
+            }
+        }
+
+        if (comptime elements) {
+            if (child.is(Element)) |el| {
+                try coll.append(allocator, el);
+            }
+        } else {
+            try coll.append(allocator, child);
+        }
+    }
+}
+
+pub fn assign(self: *Slot, nodes: []const *Node) void {
+    // Imperative slot assignment API
+    // This would require storing manually assigned nodes
+    // For now, this is a placeholder for the API
+    _ = self;
+    _ = nodes;
+
+    // let's see if this is ever actually used
+    log.warn(.not_implemented, "Slot.assign", .{ });
+}
+
+fn findShadowRoot(self: *Slot) ?*ShadowRoot {
+    // Walk up the parent chain to find the shadow root
+    var parent = self.asNode()._parent;
+    while (parent) |p| {
+        if (p.is(ShadowRoot)) |shadow_root| {
+            return shadow_root;
+        }
+        parent = p._parent;
+    }
+    return null;
+}
+
+fn isAssignedToSlot(node: *Node, slot_name: []const u8) bool {
+    // Check if a node should be assigned to a slot with the given name
+    if (node.is(Element)) |element| {
+        // Get the slot attribute from the element
+        const node_slot = element.getAttributeSafe("slot") orelse "";
+
+        // Match if:
+        // - Both are empty (default slot)
+        // - They match exactly
+        return std.mem.eql(u8, node_slot, slot_name);
+    }
+
+    // Text nodes, comments, etc. are only assigned to the default slot
+    // (when they have no preceding/following element siblings with slot attributes)
+    // For simplicity, text nodes go to default slot if slot_name is empty
+    return slot_name.len == 0;
+}
+
+pub const JsApi = struct {
+    pub const bridge = js.Bridge(Slot);
+
+    pub const Meta = struct {
+        pub const name = "HTMLSlotElement";
+        pub const prototype_chain = bridge.prototypeChain();
+        pub var class_id: bridge.ClassId = undefined;
+    };
+
+    pub const name = bridge.accessor(Slot.getName, Slot.setName, .{});
+    pub const assignedNodes = bridge.function(Slot.assignedNodes, .{});
+    pub const assignedElements = bridge.function(Slot.assignedElements, .{});
+    pub const assign = bridge.function(Slot.assign, .{});
+};
+
+const testing = @import("../../../../testing.zig");
+test "WebApi: HTMLSlotElement" {
+    try testing.htmlRunner("element/html/slot.html", .{});
+}