diff --git a/src/browser/Frame.zig b/src/browser/Frame.zig
index 59841f2b7..8ab70b904 100644
--- a/src/browser/Frame.zig
+++ b/src/browser/Frame.zig
@@ -200,7 +200,8 @@ _broadcast_channels: std.DoublyLinkedList = .{},
 // List of MessagePorts living in this frame's context.
 _message_ports: std.DoublyLinkedList = .{},
 
-// MutationObserver / IntersectionObserver bookkeeping. See frame/observers.zig.
+// See frame/observers.zig.
+_resize: observers.Resize = .{},
 _mutation: observers.Mutation = .{},
 _intersection: observers.Intersection = .{},
 
@@ -1944,14 +1945,10 @@ pub fn openPopup(self: *Frame, opts: OpenPopupOpts) !*Frame {
 pub fn domChanged(self: *Frame) void {
     self._page.dom_version += 1;
 
-    if (self._intersection.check_scheduled) {
-        return;
-    }
-
-    self._intersection.check_scheduled = true;
-    self.js.queueIntersectionChecks() catch |err| {
-        log.err(.frame, "frame.schedIntersectChecks", .{ .err = err, .type = self._type, .url = self.url });
-    };
+    // A DOM change is our "rendering opportunity": re-evaluate the layout
+    // observers. Both are no-ops unless something they track actually changed.
+    observers.scheduleIntersectionChecks(self);
+    observers.scheduleResizeDelivery(self);
 }
 
 const ElementIdMaps = struct { lookup: *std.StringHashMapUnmanaged(*Element), removed_ids: *std.StringHashMapUnmanaged(void) };
diff --git a/src/browser/frame/observers.zig b/src/browser/frame/observers.zig
index d27e5c0d6..de9defed5 100644
--- a/src/browser/frame/observers.zig
+++ b/src/browser/frame/observers.zig
@@ -30,6 +30,7 @@ const Page = @import("../Page.zig");
 const Node = @import("../webapi/Node.zig");
 const Event = @import("../webapi/Event.zig");
 const Element = @import("../webapi/Element.zig");
+const ResizeObserver = @import("../webapi/ResizeObserver.zig");
 const MutationObserver = @import("../webapi/MutationObserver.zig");
 const IntersectionObserver = @import("../webapi/IntersectionObserver.zig");
 
@@ -52,6 +53,14 @@ pub const Intersection = struct {
     delivery_scheduled: bool = false,
 };
 
+// ResizeObserver bookkeeping for a frame.
+pub const Resize = struct {
+    // List of active ResizeObservers (i.e. those with >= 1 observation)
+    observers: std.ArrayList(*ResizeObserver) = .{},
+    delivery_scheduled: bool = false,
+    delivery_depth: u32 = 0,
+};
+
 // Releases the frame's references to its registered observers. Called from
 // Frame.deinit.
 pub fn deinit(frame: *Frame, page: *Page) void {
@@ -65,6 +74,10 @@ pub fn deinit(frame: *Frame, page: *Page) void {
     for (frame._intersection.observers.items) |observer| {
         observer.releaseRef(page);
     }
+
+    for (frame._resize.observers.items) |observer| {
+        observer.releaseRef(page);
+    }
 }
 
 pub fn registerMutationObserver(frame: *Frame, observer: *MutationObserver) !void {
@@ -92,6 +105,21 @@ pub fn unregisterIntersectionObserver(frame: *Frame, observer: *IntersectionObse
     }
 }
 
+pub fn registerResizeObserver(frame: *Frame, observer: *ResizeObserver) !void {
+    observer.acquireRef();
+    try frame._resize.observers.append(frame.arena, observer);
+}
+
+pub fn unregisterResizeObserver(frame: *Frame, observer: *ResizeObserver) void {
+    for (frame._resize.observers.items, 0..) |obs, i| {
+        if (obs == observer) {
+            observer.releaseRef(frame._page);
+            _ = frame._resize.observers.swapRemove(i);
+            return;
+        }
+    }
+}
+
 pub fn hasMutationObservers(frame: *const Frame) bool {
     return frame._mutation.observers.first != null;
 }
@@ -118,6 +146,63 @@ pub fn scheduleIntersectionDelivery(frame: *Frame) !void {
     try frame.js.queueIntersectionDelivery();
 }
 
+pub fn scheduleIntersectionChecks(frame: *Frame) void {
+    if (frame._intersection.check_scheduled) {
+        return;
+    }
+    frame._intersection.check_scheduled = true;
+    frame.js.queueIntersectionChecks() catch |err| {
+        frame._intersection.check_scheduled = false;
+        log.err(.frame, "frame.scheduleIntersectionChecks", .{ .err = err, .type = frame._type, .url = frame.url });
+    };
+}
+
+pub fn scheduleResizeDelivery(frame: *Frame) void {
+    if (frame._resize.observers.items.len == 0) {
+        return;
+    }
+    if (frame._resize.delivery_scheduled) {
+        return;
+    }
+    frame._resize.delivery_scheduled = true;
+    frame.js.queueResizeDelivery() catch |err| {
+        frame._resize.delivery_scheduled = false;
+        log.err(.frame, "frame.scheduleResizeDelivery", .{ .err = err, .type = frame._type, .url = frame.url });
+    };
+}
+
+pub fn deliverResizes(frame: *Frame) void {
+    if (!frame._resize.delivery_scheduled) {
+        return;
+    }
+    frame._resize.delivery_scheduled = false;
+
+    // guard against a callback that keeps mutating the layout, and thus causes
+    // an endless stram of deliveries.
+    frame._resize.delivery_depth += 1;
+    defer if (!frame._resize.delivery_scheduled) {
+        frame._resize.delivery_depth = 0;
+    };
+    if (frame._resize.delivery_depth > 50) {
+        log.warn(.frame, "frame.ResizeLimit", .{ .type = frame._type, .url = frame.url });
+        frame._resize.delivery_depth = 0;
+        return;
+    }
+
+    // Iterate backwards so an observer disconnecting during its callback is safe.
+    var i = frame._resize.observers.items.len;
+    while (i > 0) {
+        i -= 1;
+        if (i >= frame._resize.observers.items.len) {
+            continue;
+        }
+        const observer = frame._resize.observers.items[i];
+        observer.deliverEntries(frame) catch |err| {
+            log.err(.frame, "frame.deliverResizes", .{ .err = err, .type = frame._type, .url = frame.url });
+        };
+    }
+}
+
 pub fn performScheduledIntersectionChecks(frame: *Frame) void {
     if (!frame._intersection.check_scheduled) {
         return;
@@ -157,7 +242,7 @@ pub fn deliverMutations(frame: *Frame) void {
         frame._mutation.delivery_depth = 0;
     };
 
-    if (frame._mutation.delivery_depth > 100) {
+    if (frame._mutation.delivery_depth > 50) {
         log.err(.frame, "frame.MutationLimit", .{ .type = frame._type, .url = frame.url });
         frame._mutation.delivery_depth = 0;
         return;
diff --git a/src/browser/js/Context.zig b/src/browser/js/Context.zig
index a8b341dfe..f8c5163e8 100644
--- a/src/browser/js/Context.zig
+++ b/src/browser/js/Context.zig
@@ -1093,6 +1093,17 @@ pub fn queueIntersectionDelivery(self: *Context) !void {
     }.run);
 }
 
+pub fn queueResizeDelivery(self: *Context) !void {
+    self.enqueueMicrotask(struct {
+        fn run(ctx: *Context) void {
+            switch (ctx.global) {
+                .frame => |frame| Frame.observers.deliverResizes(frame),
+                .worker => unreachable,
+            }
+        }
+    }.run);
+}
+
 pub fn queueCustomElementBackupDrain(self: *Context) !void {
     self.enqueueMicrotask(struct {
         fn run(ctx: *Context) void {
diff --git a/src/browser/tests/resize_observer/basic.html b/src/browser/tests/resize_observer/basic.html
new file mode 100644
index 000000000..4d8d8a2c1
--- /dev/null
+++ b/src/browser/tests/resize_observer/basic.html
@@ -0,0 +1,54 @@
+<!DOCTYPE html>
+<script src="../testing.js"></script>
+<div id="target" style="width: 100px; height: 60px;">Target Element</div>
+<div id="hidden" style="width: 50px; display: none;">Hidden</div>
+
+<script id="initial">
+  {
+    const target = document.getElementById('target');
+    let count = 0;
+    let entries = null;
+
+    const observer = new ResizeObserver((observerEntries, obs) => {
+        count += 1;
+        entries = observerEntries;
+    });
+    observer.observe(target);
+
+    testing.onload(() => {
+        // observe() delivers a single initial entry
+        testing.expectEqual(1, count);
+        testing.expectEqual(1, entries.length);
+
+        const entry = entries[0];
+        testing.expectEqual(target, entry.target);
+        testing.expectEqual('object', typeof entry.contentRect);
+        testing.expectEqual(100, entry.contentRect.width);
+        testing.expectEqual(60, entry.contentRect.height);
+
+        // content-box / border-box / device-pixel are the same for us
+        testing.expectEqual(1, entry.contentBoxSize.length);
+        testing.expectEqual(100, entry.contentBoxSize[0].inlineSize);
+        testing.expectEqual(60, entry.contentBoxSize[0].blockSize);
+        testing.expectEqual(100, entry.borderBoxSize[0].inlineSize);
+        testing.expectEqual(100, entry.devicePixelContentBoxSize[0].inlineSize);
+
+        observer.disconnect();
+    });
+  }
+</script>
+
+<script id="no_initial_for_zero_size">
+  {
+    // A display:none element has no box, so observe() delivers nothing.
+    const hidden = document.getElementById('hidden');
+    let hiddenCount = 0;
+    const hiddenObserver = new ResizeObserver(() => { hiddenCount += 1; });
+    hiddenObserver.observe(hidden);
+
+    testing.onload(() => {
+        testing.expectEqual(0, hiddenCount);
+        hiddenObserver.disconnect();
+    });
+  }
+</script>
diff --git a/src/browser/tests/resize_observer/resize.html b/src/browser/tests/resize_observer/resize.html
new file mode 100644
index 000000000..ed437249e
--- /dev/null
+++ b/src/browser/tests/resize_observer/resize.html
@@ -0,0 +1,55 @@
+<!DOCTYPE html>
+<script src="../testing.js"></script>
+<div id="anchor">anchor</div>
+
+<script id="style_change_delivers" type=module>
+  // Changing an element's size delivers a fresh notification.
+  const state = await testing.async();
+  const t = document.createElement('div');
+  t.style.width = '100px';
+  t.style.height = '50px';
+  document.body.appendChild(t);
+
+  let step = 0;
+  const observer = new ResizeObserver((entries) => {
+    step += 1;
+    if (step === 1) {
+      testing.expectEqual(100, entries[0].contentRect.width);
+      t.style.width = '25px';
+    } else if (step === 2) {
+      testing.expectEqual(25, entries[0].contentRect.width);
+      observer.disconnect();
+      state.resolve();
+    }
+  });
+  observer.observe(t);
+
+  await state.done(() => {
+    testing.expectEqual(2, step);
+  });
+</script>
+
+<script id="disconnect_from_dom" type=module>
+  // Removing the target from the DOM collapses its size to 0 and notifies.
+  const state = await testing.async();
+  const t = document.createElement('div');
+  t.style.width = '80px';
+  document.body.appendChild(t);
+
+  const widths = [];
+  const observer = new ResizeObserver((entries) => {
+    widths.push(entries[0].contentRect.width);
+    if (widths.length === 1) {
+      t.remove();
+    } else if (widths.length === 2) {
+      observer.disconnect();
+      state.resolve();
+    }
+  });
+  observer.observe(t);
+
+  await state.done(() => {
+    testing.expectEqual(80, widths[0]);
+    testing.expectEqual(0, widths[1]);
+  });
+</script>
diff --git a/src/browser/tests/resize_observer/unobserve.html b/src/browser/tests/resize_observer/unobserve.html
new file mode 100644
index 000000000..6c4155a07
--- /dev/null
+++ b/src/browser/tests/resize_observer/unobserve.html
@@ -0,0 +1,64 @@
+<!DOCTYPE html>
+<script src="../testing.js"></script>
+<div id="anchor">anchor</div>
+
+<script id="unobserve_before_delivery">
+  {
+    const t = document.createElement('div');
+    t.style.width = '100px';
+    document.body.appendChild(t);
+
+    let count = 0;
+    const observer = new ResizeObserver(() => { count += 1; });
+    observer.observe(t);
+    observer.unobserve(t);
+    // unobserving a target we never observed is a no-op
+    observer.unobserve(document.body);
+
+    testing.onload(() => {
+        testing.expectEqual(0, count);
+        observer.disconnect();
+    });
+  }
+</script>
+
+<script id="disconnect_stops_notifications">
+  {
+    const t1 = document.createElement('div');
+    const t2 = document.createElement('div');
+    t1.style.width = '100px';
+    t2.style.width = '100px';
+    document.body.appendChild(t1);
+    document.body.appendChild(t2);
+
+    let count = 0;
+    const observer = new ResizeObserver(() => { count += 1; });
+    observer.observe(t1);
+    observer.observe(t2);
+    observer.disconnect();
+    t1.style.width = '30px';
+
+    testing.onload(() => {
+        testing.expectEqual(0, count);
+    });
+  }
+</script>
+
+<script id="observe_twice_is_one">
+  {
+    const t = document.createElement('div');
+    t.style.width = '100px';
+    document.body.appendChild(t);
+
+    let entryCount = 0;
+    const observer = new ResizeObserver((entries) => { entryCount += entries.length; });
+    observer.observe(t);
+    observer.observe(t);
+
+    testing.onload(() => {
+        // observing the same target twice still yields a single entry
+        testing.expectEqual(1, entryCount);
+        observer.disconnect();
+    });
+  }
+</script>
diff --git a/src/browser/webapi/ResizeObserver.zig b/src/browser/webapi/ResizeObserver.zig
index 29c99003f..32fd0ebe4 100644
--- a/src/browser/webapi/ResizeObserver.zig
+++ b/src/browser/webapi/ResizeObserver.zig
@@ -1,4 +1,4 @@
-// Copyright (C) 2023-2025  Lightpanda (Selecy SAS)
+// Copyright (C) 2023-2026  Lightpanda (Selecy SAS)
 //
 // Francis Bouvier <francis@lightpanda.io>
 // Pierre Tachoire <pierre@lightpanda.io>
@@ -16,39 +16,238 @@
 // 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/>.
 
+// We're a headless browser, so this is never goig to be perfect, but we CAN
+// correctly deliver some effects, e.g. the initial entry when observe() is
+// called and some changes to display or style's width/height.
+
+const std = @import("std");
+const lp = @import("lightpanda");
+
 const js = @import("../js/js.zig");
+
+const Page = @import("../Page.zig");
+const Frame = @import("../Frame.zig");
+
 const Element = @import("Element.zig");
+const DOMRect = @import("DOMRect.zig");
+const Factory = @import("../Factory.zig");
 
-pub const ResizeObserver = @This();
+const log = lp.log;
+const Allocator = std.mem.Allocator;
 
-// Padding to avoid zero-size struct, which causes identity_map pointer collisions.
-_pad: bool = false,
-
-fn init(cbk: js.Function) ResizeObserver {
-    _ = cbk;
-    return .{};
+pub fn registerTypes() []const type {
+    return &.{
+        ResizeObserver,
+        ResizeObserverEntry,
+        ResizeObserverSize,
+    };
 }
 
+const ResizeObserver = @This();
+
+_rc: lp.RC(u8) = .{},
+_arena: Allocator,
+_callback: js.Function.Global,
+_observations: std.ArrayList(Observation) = .{},
+
+const Observation = struct {
+    target: *Element,
+    last_width: f64 = 0,
+    last_height: f64 = 0,
+};
+
 const Options = struct {
     box: []const u8 = "content-box",
 };
-pub fn observe(self: *const ResizeObserver, element: *Element, options_: ?Options) void {
-    _ = self;
-    _ = element;
-    _ = options_;
-    return;
+
+pub fn init(callback: js.Function.Global, frame: *Frame) !*ResizeObserver {
+    const arena = try frame.getArena(.small, "ResizeObserver");
+    errdefer frame.releaseArena(arena);
+
+    const self = try arena.create(ResizeObserver);
+    self.* = .{
+        ._arena = arena,
+        ._callback = callback,
+    };
+    return self;
 }
 
-pub fn unobserve(self: *const ResizeObserver, element: *Element) void {
-    _ = self;
-    _ = element;
-    return;
+pub fn deinit(self: *ResizeObserver, page: *Page) void {
+    self._callback.release();
+    page.releaseArena(self._arena);
 }
 
-pub fn disconnect(self: *const ResizeObserver) void {
-    _ = self;
+pub fn acquireRef(self: *ResizeObserver) void {
+    self._rc.acquire();
 }
 
+pub fn releaseRef(self: *ResizeObserver, page: *Page) void {
+    self._rc.release(self, page);
+}
+
+pub fn observe(self: *ResizeObserver, target: *Element, options_: ?Options, frame: *Frame) !void {
+    _ = options_; // Can't make use of this
+
+    for (self._observations.items) |obs| {
+        if (obs.target == target) {
+            return;
+        }
+    }
+
+    try self._observations.append(self._arena, .{ .target = target });
+    if (self._observations.items.len == 1) {
+        try Frame.observers.registerResizeObserver(frame, self);
+    }
+
+    Frame.observers.scheduleResizeDelivery(frame);
+}
+
+pub fn unobserve(self: *ResizeObserver, target: *Element, frame: *Frame) void {
+    for (self._observations.items, 0..) |obs, i| {
+        if (obs.target == target) {
+            _ = self._observations.swapRemove(i);
+            break;
+        }
+    }
+
+    if (self._observations.items.len == 0) {
+        Frame.observers.unregisterResizeObserver(frame, self);
+    }
+}
+
+pub fn disconnect(self: *ResizeObserver, frame: *Frame) void {
+    if (self._observations.items.len == 0) {
+        return;
+    }
+    self._observations.clearRetainingCapacity();
+    Frame.observers.unregisterResizeObserver(frame, self);
+}
+
+// Gather the observations whose size changed since the last delivery and, if
+// any, invoke the callback.
+pub fn deliverEntries(self: *ResizeObserver, frame: *Frame) !void {
+    var entries: std.ArrayList(*ResizeObserverEntry) = .empty;
+    for (self._observations.items) |*obs| {
+        const target = obs.target;
+
+        const width, const height = blk: {
+            if (obs.target.asNode().isConnected() == false) {
+                break :blk .{ 0, 0 };
+            }
+            break :blk .{ target.getClientWidth(frame), target.getClientHeight(frame) };
+        };
+
+        if (width == obs.last_width and height == obs.last_height) {
+            continue;
+        }
+        obs.last_width = width;
+        obs.last_height = height;
+
+        const entry = try ResizeObserverEntry.create(obs.target, width, height, frame._factory);
+        try entries.append(frame.call_arena, entry);
+    }
+
+    if (entries.items.len == 0) {
+        return;
+    }
+
+    var caught: js.TryCatch.Caught = undefined;
+
+    var ls: js.Local.Scope = undefined;
+    frame.js.localScope(&ls);
+    defer ls.deinit();
+
+    ls.toLocal(self._callback).tryCall(void, .{ entries.items, self }, &caught) catch |err| {
+        log.err(.frame, "ResizeObserver.deliverEntries", .{ .err = err, .caught = caught });
+        return err;
+    };
+}
+
+pub const ResizeObserverEntry = struct {
+    _target: *Element,
+    _content_rect: *DOMRect,
+    _box_size: [1]*ResizeObserverSize,
+
+    pub fn create(target: *Element, width: f64, height: f64, factory: *Factory) !*ResizeObserverEntry {
+        const content_rect = try DOMRect.create(.{ .width = width, .height = height }, factory);
+        const size = try ResizeObserverSize.create(width, height, factory);
+        return factory.create(ResizeObserverEntry{
+            ._target = target,
+            ._content_rect = content_rect,
+            ._box_size = .{size},
+        });
+    }
+
+    pub fn getTarget(self: *const ResizeObserverEntry) *Element {
+        return self._target;
+    }
+
+    pub fn getContentRect(self: *const ResizeObserverEntry) *DOMRect {
+        return self._content_rect;
+    }
+
+    pub fn getBorderBoxSize(self: *const ResizeObserverEntry) []const *ResizeObserverSize {
+        return &self._box_size;
+    }
+
+    pub fn getContentBoxSize(self: *const ResizeObserverEntry) []const *ResizeObserverSize {
+        return &self._box_size;
+    }
+
+    pub fn getDevicePixelContentBoxSize(self: *const ResizeObserverEntry) []const *ResizeObserverSize {
+        return &self._box_size;
+    }
+
+    pub const JsApi = struct {
+        pub const bridge = js.Bridge(ResizeObserverEntry);
+
+        pub const Meta = struct {
+            pub const name = "ResizeObserverEntry";
+            pub const prototype_chain = bridge.prototypeChain();
+            pub var class_id: bridge.ClassId = undefined;
+        };
+
+        pub const target = bridge.accessor(ResizeObserverEntry.getTarget, null, .{});
+        pub const contentRect = bridge.accessor(ResizeObserverEntry.getContentRect, null, .{});
+        pub const borderBoxSize = bridge.accessor(ResizeObserverEntry.getBorderBoxSize, null, .{});
+        pub const contentBoxSize = bridge.accessor(ResizeObserverEntry.getContentBoxSize, null, .{});
+        pub const devicePixelContentBoxSize = bridge.accessor(ResizeObserverEntry.getDevicePixelContentBoxSize, null, .{});
+    };
+};
+
+pub const ResizeObserverSize = struct {
+    _inline_size: f64,
+    _block_size: f64,
+
+    pub fn create(inline_size: f64, block_size: f64, factory: *Factory) !*ResizeObserverSize {
+        return factory.create(ResizeObserverSize{
+            ._inline_size = inline_size,
+            ._block_size = block_size,
+        });
+    }
+
+    pub fn getInlineSize(self: *const ResizeObserverSize) f64 {
+        return self._inline_size;
+    }
+
+    pub fn getBlockSize(self: *const ResizeObserverSize) f64 {
+        return self._block_size;
+    }
+
+    pub const JsApi = struct {
+        pub const bridge = js.Bridge(ResizeObserverSize);
+
+        pub const Meta = struct {
+            pub const name = "ResizeObserverSize";
+            pub const prototype_chain = bridge.prototypeChain();
+            pub var class_id: bridge.ClassId = undefined;
+        };
+
+        pub const inlineSize = bridge.accessor(ResizeObserverSize.getInlineSize, null, .{});
+        pub const blockSize = bridge.accessor(ResizeObserverSize.getBlockSize, null, .{});
+    };
+};
+
 pub const JsApi = struct {
     pub const bridge = js.Bridge(ResizeObserver);
 
@@ -56,7 +255,6 @@ pub const JsApi = struct {
         pub const name = "ResizeObserver";
         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(ResizeObserver.init, .{});
@@ -64,3 +262,8 @@ pub const JsApi = struct {
     pub const unobserve = bridge.function(ResizeObserver.unobserve, .{});
     pub const disconnect = bridge.function(ResizeObserver.disconnect, .{});
 };
+
+const testing = @import("../../testing.zig");
+test "WebApi: ResizeObserver" {
+    try testing.htmlRunner("resize_observer", .{});
+}