diff --git a/src/browser/Frame.zig b/src/browser/Frame.zig
index d537c2df6..0fea2ba7c 100644
--- a/src/browser/Frame.zig
+++ b/src/browser/Frame.zig
@@ -147,6 +147,8 @@ _queued_events_1: std.ArrayList(QueuedEvent) = .empty,
 _queued_events_2: std.ArrayList(QueuedEvent) = .empty,
 _queued_events: *std.ArrayList(QueuedEvent) = undefined,
 
+_focus_fixup_pending: bool = false,
+
 _style_manager: StyleManager,
 _script_manager: ScriptManager,
 
@@ -2369,6 +2371,25 @@ pub fn queueElementEvent(self: *Frame, element: *Element.Html, kind: QueuedEvent
     }
 }
 
+// An element that becomes inert can't stay focused. Fire its blur on the next tick
+fn scheduleFocusFixup(self: *Frame) !void {
+    if (self._focus_fixup_pending or self.document._active_element == null) {
+        return;
+    }
+    try self.js.scheduler.add(self, struct {
+        fn run(ctx: *anyopaque) !?u32 {
+            const f: *Frame = @ptrCast(@alignCast(ctx));
+            f._focus_fixup_pending = false;
+            const active = f.document._active_element orelse return null;
+            if (active.asNode().isInert(f)) {
+                try active.blur(f);
+            }
+            return null;
+        }
+    }.run, 5, .{ .name = "frame.focusFixup" });
+    self._focus_fixup_pending = true;
+}
+
 const HashChangeCallback = struct {
     frame: *Frame,
     old_url: []const u8,
@@ -3148,6 +3169,10 @@ pub fn attributeChange(self: *Frame, element: *Element, name: String, value: Str
     } else if (name.eql(comptime .wrap("style"))) {
         element._flags.has_inline_style = true;
         self.styleAttributeChanged(element, value.str());
+    } else if (name.eql(comptime .wrap("inert"))) {
+        self.scheduleFocusFixup() catch |err| {
+            log.err(.frame, "scheduleFocusFixup", .{ .err = err, .type = self._type, .url = self.url });
+        };
     }
 }
 
diff --git a/src/browser/tests/element/inert.html b/src/browser/tests/element/inert.html
new file mode 100644
index 000000000..408b887cd
--- /dev/null
+++ b/src/browser/tests/element/inert.html
@@ -0,0 +1,102 @@
+<!DOCTYPE html>
+<script src="../testing.js"></script>
+<body>
+  <button id="outside">outside</button>
+  <div id="container" inert>
+    <input id="inner" type="text">
+    <span id="text">hidden</span>
+  </div>
+  <div id="selectable">
+    <p>shown</p>
+    <span inert>hidden</span>
+    <math inert><mi>math</mi></math>
+  </div>
+</body>
+
+<script id="inert_property">
+{
+  testing.expectEqual(false, $('#outside').inert);
+  testing.expectEqual(true, $('#container').inert);
+  // inherited inertness isn't reflected
+  testing.expectEqual(false, $('#inner').inert);
+
+  const outside = $('#outside');
+  outside.inert = true;
+  testing.expectEqual('', outside.getAttribute('inert'));
+  outside.inert = false;
+  testing.expectEqual(false, outside.hasAttribute('inert'));
+}
+</script>
+
+<script id="inert_unfocusable">
+{
+  const outside = $('#outside');
+  outside.focus();
+  testing.expectEqual(outside, document.activeElement);
+
+  $('#inner').focus();
+  testing.expectEqual(outside, document.activeElement);
+
+  $('#container').inert = false;
+  $('#inner').focus();
+  testing.expectEqual($('#inner'), document.activeElement);
+  $('#container').inert = true;
+}
+</script>
+
+<script id="inert_selection">
+{
+  const selection = getSelection();
+  selection.selectAllChildren($('#selectable'));
+  // inert only applies to HTML elements, so the MathML text stays
+  testing.expectEqual('shown math', selection.toString().trim().replace(/\s+/g, ' '));
+  selection.removeAllRanges();
+}
+</script>
+
+<script id="inert_focus_fixup" type=module>
+  const state = await testing.async();
+  const nextRendering = () => new Promise(resolve => requestAnimationFrame(() => requestAnimationFrame(resolve)));
+
+  const outside = $('#outside');
+  outside.focus();
+  const events = [];
+  outside.addEventListener('blur', () => events.push('blur'));
+  outside.addEventListener('focusout', () => events.push('focusout'));
+  outside.inert = true;
+  // like browsers, the fixup waits for the next rendering update
+  const syncActive = document.activeElement;
+  await nextRendering();
+  const fixedActive = document.activeElement;
+  outside.inert = false;
+
+  const host = document.createElement('div');
+  host.innerHTML = '<button id="slotted" slot="s">slotted</button>';
+  document.body.appendChild(host);
+  const shadow = host.attachShadow({ mode: 'open' });
+  const slot = document.createElement('slot');
+  slot.name = 's';
+  slot.inert = true;
+  shadow.appendChild(slot);
+  const shadowButton = document.createElement('button');
+  shadow.appendChild(shadowButton);
+
+  $('#slotted').focus();
+  const slottedActive = document.activeElement;
+  shadowButton.focus();
+  const shadowActive = document.activeElement;
+  host.inert = true;
+  await nextRendering();
+  const hostFixedActive = document.activeElement;
+  host.remove();
+
+  state.resolve();
+  await state.done(() => {
+    testing.expectEqual(outside, syncActive);
+    testing.expectEqual(document.body, fixedActive);
+    testing.expectEqual(['blur', 'focusout'], events);
+    testing.expectEqual(document.body, slottedActive);
+    testing.expectEqual(host, shadowActive);
+    testing.expectEqual(document.body, hostFixedActive);
+  });
+</script>
diff --git a/src/browser/webapi/Element.zig b/src/browser/webapi/Element.zig
index fded393c2..4283c67ba 100644
--- a/src/browser/webapi/Element.zig
+++ b/src/browser/webapi/Element.zig
@@ -1233,12 +1233,17 @@ pub fn focusTabIndex(self: *Element) ?i32 {
     return null;
 }
 
-// A focusable area that can take focus right now: connected and being rendered.
+// A focusable area that can take focus right now: connected, not inert and
+// being rendered.
 pub fn isFocusable(self: *Element, frame: *Frame) bool {
     if (self.focusTabIndex() == null) {
         return false;
     }
-    if (self.asNode().isConnected() == false) {
+    const node = self.asNode();
+    if (node.isConnected() == false) {
+        return false;
+    }
+    if (node.isInert(frame)) {
         return false;
     }
     return self.isVisible(frame);
diff --git a/src/browser/webapi/Node.zig b/src/browser/webapi/Node.zig
index d75d7bfbc..364178b6d 100644
--- a/src/browser/webapi/Node.zig
+++ b/src/browser/webapi/Node.zig
@@ -1699,6 +1699,27 @@ pub fn assignedSlot(self: *Node, frame: *const Frame) ?*Element.Html.Slot {
     return frame.page._assigned_slots.get(self);
 }
 
+// An inert element applies to all its chidren, so walk up to see if we have
+// an inert parent
+pub fn isInert(self: *Node, frame: *const Frame) bool {
+    var current: ?*Node = self;
+    while (current) |node| {
+        if (node.is(Element)) |el| {
+            if (el._namespace == .html and el.hasAttributeSafe(comptime .wrap("inert"))) {
+                return true;
+            }
+        }
+        if (node.assignedSlot(frame)) |slot| {
+            current = slot.asNode();
+        } else if (node.is(ShadowRoot)) |shadow| {
+            current = shadow._host.asNode();
+        } else {
+            current = node._parent;
+        }
+    }
+    return false;
+}
+
 pub const JsApi = struct {
     pub const bridge = js.Bridge(Node);
 
diff --git a/src/browser/webapi/Range.zig b/src/browser/webapi/Range.zig
index 349837467..22b1b316f 100644
--- a/src/browser/webapi/Range.zig
+++ b/src/browser/webapi/Range.zig
@@ -719,11 +719,19 @@ fn createContextualFragment(self: *const Range, html: []const u8, frame: *Frame)
 pub fn toString(self: *const Range, frame: *Frame) ![]const u8 {
     // Simplified implementation: just extract text content
     var buf = std.Io.Writer.Allocating.init(frame.local_arena);
-    try self.writeTextContent(&buf.writer);
+    try self.writeTextContent(&buf.writer, null);
     return buf.written();
 }
 
-fn writeTextContent(self: *const Range, writer: *std.Io.Writer) !void {
+// Selection.toString is almost like Range.toString, except it does not include
+// the text that an inert element hides.
+pub fn toSelectionString(self: *const Range, frame: *Frame) ![]const u8 {
+    var buf = std.Io.Writer.Allocating.init(frame.local_arena);
+    try self.writeTextContent(&buf.writer, frame);
+    return buf.written();
+}
+
+fn writeTextContent(self: *const Range, writer: *std.Io.Writer, skip_inert: ?*const Frame) !void {
     if (self._proto.getCollapsed()) return;
 
     const start_node = self._proto._start_container;
@@ -734,7 +742,7 @@ fn writeTextContent(self: *const Range, writer: *std.Io.Writer) !void {
     // Same text node — just substring
     if (start_node == end_node) {
         if (start_node.is(Node.CData)) |cdata| {
-            if (!isCommentOrPI(cdata)) {
+            if (includeText(cdata, skip_inert)) {
                 const data = cdata.getData().str();
                 const s = byteOffset(data, start_offset);
                 const e = byteOffset(data, end_offset);
@@ -748,7 +756,7 @@ fn writeTextContent(self: *const Range, writer: *std.Io.Writer) !void {
 
     // Partial start: if start container is a text node, write from offset to end
     if (start_node.is(Node.CData)) |cdata| {
-        if (!isCommentOrPI(cdata)) {
+        if (includeText(cdata, skip_inert)) {
             const data = cdata.getData().str();
             const s = byteOffset(data, start_offset);
             try writer.writeAll(data[s..]);
@@ -775,7 +783,7 @@ fn writeTextContent(self: *const Range, writer: *std.Io.Writer) !void {
                 if (n == we) break;
             }
             if (n.is(Node.CData)) |cdata| {
-                if (!isCommentOrPI(cdata)) {
+                if (includeText(cdata, skip_inert)) {
                     try writer.writeAll(cdata.getData().str());
                 }
             }
@@ -786,7 +794,7 @@ fn writeTextContent(self: *const Range, writer: *std.Io.Writer) !void {
     // Partial end: if end container is a different text node, write from start to offset
     if (start_node != end_node) {
         if (end_node.is(Node.CData)) |cdata| {
-            if (!isCommentOrPI(cdata)) {
+            if (includeText(cdata, skip_inert)) {
                 const data = cdata.getData().str();
                 const e = byteOffset(data, end_offset);
                 try writer.writeAll(data[0..e]);
@@ -795,6 +803,14 @@ fn writeTextContent(self: *const Range, writer: *std.Io.Writer) !void {
     }
 }
 
+fn includeText(cdata: *Node.CData, skip_inert: ?*const Frame) bool {
+    if (isCommentOrPI(cdata)) {
+        return false;
+    }
+    const frame = skip_inert orelse return true;
+    return cdata.asNode().isInert(frame) == false;
+}
+
 fn isCommentOrPI(cdata: *Node.CData) bool {
     return cdata.is(Node.CData.Comment) != null or cdata.is(Node.CData.ProcessingInstruction) != null;
 }
diff --git a/src/browser/webapi/Selection.zig b/src/browser/webapi/Selection.zig
index 41cb692bf..7124f1655 100644
--- a/src/browser/webapi/Selection.zig
+++ b/src/browser/webapi/Selection.zig
@@ -714,7 +714,7 @@ pub fn collapse(self: *Selection, _node: ?*Node, _offset: ?u32, frame: *Frame) !
 
 pub fn toString(self: *const Selection, frame: *Frame) ![]const u8 {
     const range = self._range orelse return "";
-    return try range.toString(frame);
+    return try range.toSelectionString(frame);
 }
 
 fn setRange(self: *Selection, new_range: ?*Range, frame: *Frame) void {
diff --git a/src/browser/webapi/element/Html.zig b/src/browser/webapi/element/Html.zig
index 51637d7e2..a671386f0 100644
--- a/src/browser/webapi/element/Html.zig
+++ b/src/browser/webapi/element/Html.zig
@@ -452,6 +452,18 @@ pub fn setHidden(self: *HtmlElement, hidden: bool, frame: *Frame) !void {
     }
 }
 
+pub fn getInert(self: *HtmlElement) bool {
+    return self.asElement().hasAttributeSafe(comptime .wrap("inert"));
+}
+
+pub fn setInert(self: *HtmlElement, inert: bool, frame: *Frame) !void {
+    if (inert) {
+        try self.asElement().setAttributeSafe(comptime .wrap("inert"), .wrap(""), frame);
+    } else {
+        try self.asElement().removeAttribute(comptime .wrap("inert"), frame);
+    }
+}
+
 // The translate IDL attribute reflects the element's translation mode:
 // translate="yes"/"" enables it, "no" disables it, anything else (or no
 // attribute) inherits from the parent, defaulting to enabled.
@@ -1862,6 +1874,7 @@ pub const JsApi = struct {
     pub const dir = reflect.enumerated("dir", &.{ "ltr", "rtl", "auto" }, .{});
     pub const draggable = bridge.accessor(HtmlElement.getDraggable, HtmlElement.setDraggable, .{ .ce_reactions = true });
     pub const hidden = bridge.accessor(HtmlElement.getHidden, HtmlElement.setHidden, .{ .ce_reactions = true });
+    pub const inert = bridge.accessor(HtmlElement.getInert, HtmlElement.setInert, .{ .ce_reactions = true });
     pub const translate = bridge.accessor(HtmlElement.getTranslate, HtmlElement.setTranslate, .{ .ce_reactions = true });
     pub const accessKeyLabel = bridge.accessor(HtmlElement.getAccessKeyLabel, null, .{});
     pub const popover = bridge.accessor(HtmlElement.getPopover, HtmlElement.setPopover, .{ .ce_reactions = true });