diff --git a/packages/mui-base/src/Select/Arrow/SelectArrow.test.tsx b/packages/mui-base/src/Select/Arrow/SelectArrow.test.tsx
index 1ccf28a15..e1da70572 100644
--- a/packages/mui-base/src/Select/Arrow/SelectArrow.test.tsx
+++ b/packages/mui-base/src/Select/Arrow/SelectArrow.test.tsx
@@ -1,18 +1,18 @@
-// import * as React from 'react';
-// import { Select } from '@base_ui/react/Select';
-// import { createRenderer, describeConformance } from '#test-utils';
+import * as React from 'react';
+import { Select } from '@base_ui/react/Select';
+import { createRenderer, describeConformance } from '#test-utils';
 
-// describe('<Select.Arrow />', () => {
-//   const { render } = createRenderer();
+describe('<Select.Arrow />', () => {
+  const { render } = createRenderer();
 
-//   describeConformance(<Select.Arrow />, () => ({
-//     refInstanceof: window.HTMLDivElement,
-//     render(node) {
-//       return render(
-//         <Select.Root open animated={false} alignOptionToTrigger={false}>
-//           <Select.Positioner>{node}</Select.Positioner>
-//         </Select.Root>,
-//       );
-//     },
-//   }));
-// });
+  describeConformance(<Select.Arrow />, () => ({
+    refInstanceof: window.HTMLDivElement,
+    render(node) {
+      return render(
+        <Select.Root open animated={false} alignOptionToTrigger={false}>
+          <Select.Positioner>{node}</Select.Positioner>
+        </Select.Root>,
+      );
+    },
+  }));
+});
diff --git a/packages/mui-base/src/Select/Backdrop/SelectBackdrop.test.tsx b/packages/mui-base/src/Select/Backdrop/SelectBackdrop.test.tsx
index a06c83d9a..3d8251c77 100644
--- a/packages/mui-base/src/Select/Backdrop/SelectBackdrop.test.tsx
+++ b/packages/mui-base/src/Select/Backdrop/SelectBackdrop.test.tsx
@@ -1,18 +1,18 @@
-// import * as React from 'react';
-// import { Select } from '@base_ui/react/Select';
-// import { createRenderer, describeConformance } from '#test-utils';
+import * as React from 'react';
+import { Select } from '@base_ui/react/Select';
+import { createRenderer, describeConformance } from '#test-utils';
 
-// describe('<Select.Backdrop />', () => {
-//   const { render } = createRenderer();
+describe('<Select.Backdrop />', () => {
+  const { render } = createRenderer();
 
-//   describeConformance(<Select.Backdrop />, () => ({
-//     refInstanceof: window.HTMLDivElement,
-//     render(node) {
-//       return render(
-//         <Select.Root open animated={false}>
-//           {node}
-//         </Select.Root>,
-//       );
-//     },
-//   }));
-// });
+  describeConformance(<Select.Backdrop />, () => ({
+    refInstanceof: window.HTMLDivElement,
+    render(node) {
+      return render(
+        <Select.Root open animated={false}>
+          {node}
+        </Select.Root>,
+      );
+    },
+  }));
+});
diff --git a/packages/mui-base/src/Select/Group/SelectGroup.test.tsx b/packages/mui-base/src/Select/Group/SelectGroup.test.tsx
index 0017235ae..a9a673870 100644
--- a/packages/mui-base/src/Select/Group/SelectGroup.test.tsx
+++ b/packages/mui-base/src/Select/Group/SelectGroup.test.tsx
@@ -1,55 +1,55 @@
-// import * as React from 'react';
-// import { Select } from '@base_ui/react/Select';
-// import { createRenderer, describeConformance } from '#test-utils';
-// import { screen } from '@mui/internal-test-utils';
-// import { expect } from 'chai';
+import * as React from 'react';
+import { Select } from '@base_ui/react/Select';
+import { createRenderer, describeConformance } from '#test-utils';
+import { screen } from '@mui/internal-test-utils';
+import { expect } from 'chai';
 
-// describe('<Select.Group />', () => {
-//   const { render } = createRenderer();
+describe('<Select.Group />', () => {
+  const { render } = createRenderer();
 
-//   describeConformance(<Select.Group />, () => ({
-//     refInstanceof: window.HTMLDivElement,
-//     render(node) {
-//       return render(
-//         <Select.Root open animated={false}>
-//           {node}
-//         </Select.Root>,
-//       );
-//     },
-//   }));
+  describeConformance(<Select.Group />, () => ({
+    refInstanceof: window.HTMLDivElement,
+    render(node) {
+      return render(
+        <Select.Root open animated={false}>
+          {node}
+        </Select.Root>,
+      );
+    },
+  }));
 
-//   it('should render option group with label', async () => {
-//     await render(
-//       <Select.Root open animated={false}>
-//         <Select.Positioner>
-//           <Select.Group>
-//             <Select.GroupLabel>Fruits</Select.GroupLabel>
-//             <Select.Option value="apple">Apple</Select.Option>
-//             <Select.Option value="banana">Banana</Select.Option>
-//           </Select.Group>
-//         </Select.Positioner>
-//       </Select.Root>,
-//     );
+  it('should render option group with label', async () => {
+    await render(
+      <Select.Root open animated={false}>
+        <Select.Positioner>
+          <Select.Group>
+            <Select.GroupLabel>Fruits</Select.GroupLabel>
+            <Select.Option value="apple">Apple</Select.Option>
+            <Select.Option value="banana">Banana</Select.Option>
+          </Select.Group>
+        </Select.Positioner>
+      </Select.Root>,
+    );
 
-//     expect(screen.getByRole('group')).to.have.attribute('aria-labelledby');
-//     expect(screen.getByText('Fruits')).toBeVisible();
-//   });
+    expect(screen.getByRole('group')).to.have.attribute('aria-labelledby');
+    expect(screen.getByText('Fruits')).toBeVisible();
+  });
 
-//   it('should associate label with option group', async () => {
-//     await render(
-//       <Select.Root open animated={false}>
-//         <Select.Positioner>
-//           <Select.Group>
-//             <Select.GroupLabel>Vegetables</Select.GroupLabel>
-//             <Select.Option value="carrot">Carrot</Select.Option>
-//             <Select.Option value="lettuce">Lettuce</Select.Option>
-//           </Select.Group>
-//         </Select.Positioner>
-//       </Select.Root>,
-//     );
+  it('should associate label with option group', async () => {
+    await render(
+      <Select.Root open animated={false}>
+        <Select.Positioner>
+          <Select.Group>
+            <Select.GroupLabel>Vegetables</Select.GroupLabel>
+            <Select.Option value="carrot">Carrot</Select.Option>
+            <Select.Option value="lettuce">Lettuce</Select.Option>
+          </Select.Group>
+        </Select.Positioner>
+      </Select.Root>,
+    );
 
-//     const Group = screen.getByRole('group');
-//     const label = screen.getByText('Vegetables');
-//     expect(Group).to.have.attribute('aria-labelledby', label.id);
-//   });
-// });
+    const Group = screen.getByRole('group');
+    const label = screen.getByText('Vegetables');
+    expect(Group).to.have.attribute('aria-labelledby', label.id);
+  });
+});
diff --git a/packages/mui-base/src/Select/GroupLabel/SelectGroupLabel.test.tsx b/packages/mui-base/src/Select/GroupLabel/SelectGroupLabel.test.tsx
index 9a26f59c5..85f838786 100644
--- a/packages/mui-base/src/Select/GroupLabel/SelectGroupLabel.test.tsx
+++ b/packages/mui-base/src/Select/GroupLabel/SelectGroupLabel.test.tsx
@@ -1,18 +1,18 @@
-// import * as React from 'react';
-// import { Select } from '@base_ui/react/Select';
-// import { createRenderer, describeConformance } from '#test-utils';
+import * as React from 'react';
+import { Select } from '@base_ui/react/Select';
+import { createRenderer, describeConformance } from '#test-utils';
 
-// describe('<Select.GroupLabel />', () => {
-//   const { render } = createRenderer();
+describe('<Select.GroupLabel />', () => {
+  const { render } = createRenderer();
 
-//   describeConformance(<Select.GroupLabel />, () => ({
-//     refInstanceof: window.HTMLDivElement,
-//     render(node) {
-//       return render(
-//         <Select.Root open animated={false}>
-//           <Select.Group>{node}</Select.Group>
-//         </Select.Root>,
-//       );
-//     },
-//   }));
-// });
+  describeConformance(<Select.GroupLabel />, () => ({
+    refInstanceof: window.HTMLDivElement,
+    render(node) {
+      return render(
+        <Select.Root open animated={false}>
+          <Select.Group>{node}</Select.Group>
+        </Select.Root>,
+      );
+    },
+  }));
+});
diff --git a/packages/mui-base/src/Select/Icon/SelectIcon.test.tsx b/packages/mui-base/src/Select/Icon/SelectIcon.test.tsx
index 4283e54b1..1ab6b6f44 100644
--- a/packages/mui-base/src/Select/Icon/SelectIcon.test.tsx
+++ b/packages/mui-base/src/Select/Icon/SelectIcon.test.tsx
@@ -1,18 +1,18 @@
-// import * as React from 'react';
-// import { Select } from '@base_ui/react/Select';
-// import { createRenderer, describeConformance } from '#test-utils';
+import * as React from 'react';
+import { Select } from '@base_ui/react/Select';
+import { createRenderer, describeConformance } from '#test-utils';
 
-// describe('<Select.Icon />', () => {
-//   const { render } = createRenderer();
+describe('<Select.Icon />', () => {
+  const { render } = createRenderer();
 
-//   describeConformance(<Select.Icon />, () => ({
-//     refInstanceof: window.HTMLSpanElement,
-//     render(node) {
-//       return render(
-//         <Select.Root open animated={false}>
-//           {node}
-//         </Select.Root>,
-//       );
-//     },
-//   }));
-// });
+  describeConformance(<Select.Icon />, () => ({
+    refInstanceof: window.HTMLSpanElement,
+    render(node) {
+      return render(
+        <Select.Root open animated={false}>
+          {node}
+        </Select.Root>,
+      );
+    },
+  }));
+});
diff --git a/packages/mui-base/src/Select/Option/SelectOption.test.tsx b/packages/mui-base/src/Select/Option/SelectOption.test.tsx
index 87dcab572..b91100e4d 100644
--- a/packages/mui-base/src/Select/Option/SelectOption.test.tsx
+++ b/packages/mui-base/src/Select/Option/SelectOption.test.tsx
@@ -1,262 +1,262 @@
-// import * as React from 'react';
-// import { Select } from '@base_ui/react/Select';
-// import { fireEvent, flushMicrotasks, screen, waitFor } from '@mui/internal-test-utils';
-// import { createRenderer, describeConformance } from '#test-utils';
-// import { expect } from 'chai';
-
-// describe('<Select.Option />', () => {
-//   const { render } = createRenderer();
-
-//   describeConformance(<Select.Option value="" />, () => ({
-//     refInstanceof: window.HTMLDivElement,
-//     render(node) {
-//       return render(
-//         <Select.Root open animated={false}>
-//           {node}
-//         </Select.Root>,
-//       );
-//     },
-//   }));
-
-//   it('should select the option and close popup when clicked', async () => {
-//     await render(
-//       <Select.Root animated={false}>
-//         <Select.Trigger data-testid="trigger">
-//           <Select.Value placeholder="null" data-testid="value" />
-//         </Select.Trigger>
-//         <Select.Positioner data-testid="positioner">
-//           <Select.Option value="one">one</Select.Option>
-//         </Select.Positioner>
-//       </Select.Root>,
-//     );
-
-//     const value = screen.getByTestId('value');
-//     const trigger = screen.getByTestId('trigger');
-//     const positioner = screen.getByTestId('positioner');
-
-//     expect(value.textContent).to.equal('null');
-
-//     fireEvent.click(trigger);
-
-//     await flushMicrotasks();
-
-//     fireEvent.click(screen.getByText('one'));
-
-//     expect(value.textContent).to.equal('one');
-
-//     expect(positioner).not.toBeVisible();
-//   });
-
-//   it('navigating with keyboard should highlight option', async () => {
-//     const { user } = await render(
-//       <Select.Root animated={false}>
-//         <Select.Trigger data-testid="trigger">
-//           <Select.Value />
-//         </Select.Trigger>
-//         <Select.Positioner>
-//           <Select.Popup>
-//             <Select.Option value="one">one</Select.Option>
-//             <Select.Option value="two">two</Select.Option>
-//             <Select.Option value="three">three</Select.Option>
-//           </Select.Popup>
-//         </Select.Positioner>
-//       </Select.Root>,
-//     );
-
-//     const trigger = screen.getByTestId('trigger');
-
-//     fireEvent.click(trigger);
-
-//     await flushMicrotasks();
-
-//     await waitFor(() => {
-//       expect(screen.getByText('one')).toHaveFocus();
-//     });
-
-//     await user.keyboard('{ArrowDown}');
-
-//     await waitFor(() => {
-//       expect(screen.getByText('two')).toHaveFocus();
-//     });
-//   });
-
-//   it('should select option when Enter key is pressed', async function test() {
-//     if (!/jsdom/.test(window.navigator.userAgent)) {
-//       this.skip();
-//     }
-
-//     const { user } = await render(
-//       <Select.Root animated={false}>
-//         <Select.Trigger data-testid="trigger">
-//           <Select.Value placeholder="null" data-testid="value" />
-//         </Select.Trigger>
-//         <Select.Positioner>
-//           <Select.Popup>
-//             <Select.Option value="one">one</Select.Option>
-//             <Select.Option value="two">two</Select.Option>
-//           </Select.Popup>
-//         </Select.Positioner>
-//       </Select.Root>,
-//     );
-
-//     const trigger = screen.getByTestId('trigger');
-//     const value = screen.getByTestId('value');
-
-//     fireEvent.click(trigger);
-
-//     await flushMicrotasks();
-
-//     await user.keyboard('{ArrowDown}');
-//     await user.keyboard('{Enter}');
-
-//     await waitFor(() => {
-//       expect(value.textContent).to.equal('two');
-//     });
-//   });
-
-//   it('should not select disabled option', async () => {
-//     await render(
-//       <Select.Root animated={false}>
-//         <Select.Trigger data-testid="trigger">
-//           <Select.Value data-testid="value" />
-//         </Select.Trigger>
-//         <Select.Positioner>
-//           <Select.Popup>
-//             <Select.Option value="one">one</Select.Option>
-//             <Select.Option value="two" disabled>
-//               two
-//             </Select.Option>
-//           </Select.Popup>
-//         </Select.Positioner>
-//       </Select.Root>,
-//     );
-
-//     const trigger = screen.getByTestId('trigger');
-//     const value = screen.getByTestId('value');
-
-//     fireEvent.click(trigger);
-
-//     await flushMicrotasks();
-
-//     fireEvent.click(screen.getByText('two'));
-
-//     expect(value.textContent).to.equal('');
-//   });
-
-//   it('should focus the selected option upon opening the popup', async () => {
-//     const { user } = await render(
-//       <Select.Root animated={false}>
-//         <Select.Trigger data-testid="trigger">
-//           <Select.Value data-testid="value" />
-//         </Select.Trigger>
-//         <Select.Positioner>
-//           <Select.Popup>
-//             <Select.Option value="one">one</Select.Option>
-//             <Select.Option value="two">two</Select.Option>
-//             <Select.Option value="three">three</Select.Option>
-//           </Select.Popup>
-//         </Select.Positioner>
-//       </Select.Root>,
-//     );
-
-//     const trigger = screen.getByTestId('trigger');
-
-//     fireEvent.click(trigger);
-
-//     await flushMicrotasks();
-
-//     await waitFor(() => {
-//       expect(screen.getByRole('option', { name: 'one' })).toHaveFocus();
-//     });
-
-//     await user.keyboard('{ArrowDown}');
-//     await user.keyboard('{ArrowUp}');
-//     await user.keyboard('{ArrowUp}');
-
-//     fireEvent.click(screen.getByRole('option', { name: 'three' }));
-//     fireEvent.click(trigger);
-
-//     await flushMicrotasks();
-
-//     await waitFor(() => {
-//       expect(screen.getByRole('option', { name: 'three', hidden: false })).toHaveFocus();
-//     });
-//   });
-
-//   describe('style hooks', () => {
-//     it('should apply data-highlighted attribute when option is highlighted', async () => {
-//       const { user } = await render(
-//         <Select.Root animated={false}>
-//           <Select.Trigger data-testid="trigger" />
-//           <Select.Positioner>
-//             <Select.Popup>
-//               <Select.Option value="a">a</Select.Option>
-//               <Select.Option value="b">b</Select.Option>
-//             </Select.Popup>
-//           </Select.Positioner>
-//         </Select.Root>,
-//       );
-
-//       const trigger = screen.getByTestId('trigger');
-
-//       fireEvent.click(trigger);
-
-//       await flushMicrotasks();
-
-//       expect(screen.getByRole('option', { name: 'a' })).to.have.attribute(
-//         'data-highlighted',
-//         'true',
-//       );
-//       expect(screen.getByRole('option', { name: 'b' })).not.to.have.attribute('data-highlighted');
-
-//       await user.keyboard('{ArrowDown}');
-
-//       expect(screen.getByRole('option', { name: 'b' })).to.have.attribute(
-//         'data-highlighted',
-//         'true',
-//       );
-//       expect(screen.getByRole('option', { name: 'a' })).not.to.have.attribute('data-highlighted');
-//     });
-
-//     it('should apply data-selected attribute when option is selected', async () => {
-//       await render(
-//         <Select.Root animated={false}>
-//           <Select.Trigger data-testid="trigger" />
-//           <Select.Positioner>
-//             <Select.Popup>
-//               <Select.Option value="a">a</Select.Option>
-//               <Select.Option value="b">b</Select.Option>
-//             </Select.Popup>
-//           </Select.Positioner>
-//         </Select.Root>,
-//       );
-
-//       const trigger = screen.getByTestId('trigger');
-
-//       fireEvent.click(trigger);
-
-//       await flushMicrotasks();
-
-//       expect(screen.getByRole('option', { name: 'a', hidden: false })).not.to.have.attribute(
-//         'data-selected',
-//       );
-//       expect(screen.getByRole('option', { name: 'b', hidden: false })).not.to.have.attribute(
-//         'data-selected',
-//       );
-
-//       fireEvent.click(screen.getByRole('option', { name: 'a', hidden: false }));
-
-//       fireEvent.click(trigger);
-
-//       await flushMicrotasks();
-
-//       expect(screen.getByRole('option', { name: 'a', hidden: false })).to.have.attribute(
-//         'data-selected',
-//         'true',
-//       );
-//       expect(screen.getByRole('option', { name: 'b', hidden: false })).not.to.have.attribute(
-//         'data-selected',
-//       );
-//     });
-//   });
-// });
+import * as React from 'react';
+import { Select } from '@base_ui/react/Select';
+import { fireEvent, flushMicrotasks, screen, waitFor } from '@mui/internal-test-utils';
+import { createRenderer, describeConformance } from '#test-utils';
+import { expect } from 'chai';
+
+describe('<Select.Option />', () => {
+  const { render } = createRenderer();
+
+  describeConformance(<Select.Option value="" />, () => ({
+    refInstanceof: window.HTMLDivElement,
+    render(node) {
+      return render(
+        <Select.Root open animated={false}>
+          {node}
+        </Select.Root>,
+      );
+    },
+  }));
+
+  it('should select the option and close popup when clicked', async () => {
+    await render(
+      <Select.Root animated={false}>
+        <Select.Trigger data-testid="trigger">
+          <Select.Value placeholder="null" data-testid="value" />
+        </Select.Trigger>
+        <Select.Positioner data-testid="positioner">
+          <Select.Option value="one">one</Select.Option>
+        </Select.Positioner>
+      </Select.Root>,
+    );
+
+    const value = screen.getByTestId('value');
+    const trigger = screen.getByTestId('trigger');
+    const positioner = screen.getByTestId('positioner');
+
+    expect(value.textContent).to.equal('null');
+
+    fireEvent.click(trigger);
+
+    await flushMicrotasks();
+
+    fireEvent.click(screen.getByText('one'));
+
+    expect(value.textContent).to.equal('one');
+
+    expect(positioner).not.toBeVisible();
+  });
+
+  it('navigating with keyboard should highlight option', async () => {
+    const { user } = await render(
+      <Select.Root animated={false}>
+        <Select.Trigger data-testid="trigger">
+          <Select.Value />
+        </Select.Trigger>
+        <Select.Positioner>
+          <Select.Popup>
+            <Select.Option value="one">one</Select.Option>
+            <Select.Option value="two">two</Select.Option>
+            <Select.Option value="three">three</Select.Option>
+          </Select.Popup>
+        </Select.Positioner>
+      </Select.Root>,
+    );
+
+    const trigger = screen.getByTestId('trigger');
+
+    fireEvent.click(trigger);
+
+    await flushMicrotasks();
+
+    await waitFor(() => {
+      expect(screen.getByText('one')).toHaveFocus();
+    });
+
+    await user.keyboard('{ArrowDown}');
+
+    await waitFor(() => {
+      expect(screen.getByText('two')).toHaveFocus();
+    });
+  });
+
+  it('should select option when Enter key is pressed', async function test() {
+    if (!/jsdom/.test(window.navigator.userAgent)) {
+      this.skip();
+    }
+
+    const { user } = await render(
+      <Select.Root animated={false}>
+        <Select.Trigger data-testid="trigger">
+          <Select.Value placeholder="null" data-testid="value" />
+        </Select.Trigger>
+        <Select.Positioner>
+          <Select.Popup>
+            <Select.Option value="one">one</Select.Option>
+            <Select.Option value="two">two</Select.Option>
+          </Select.Popup>
+        </Select.Positioner>
+      </Select.Root>,
+    );
+
+    const trigger = screen.getByTestId('trigger');
+    const value = screen.getByTestId('value');
+
+    fireEvent.click(trigger);
+
+    await flushMicrotasks();
+
+    await user.keyboard('{ArrowDown}');
+    await user.keyboard('{Enter}');
+
+    await waitFor(() => {
+      expect(value.textContent).to.equal('two');
+    });
+  });
+
+  it('should not select disabled option', async () => {
+    await render(
+      <Select.Root animated={false}>
+        <Select.Trigger data-testid="trigger">
+          <Select.Value data-testid="value" />
+        </Select.Trigger>
+        <Select.Positioner>
+          <Select.Popup>
+            <Select.Option value="one">one</Select.Option>
+            <Select.Option value="two" disabled>
+              two
+            </Select.Option>
+          </Select.Popup>
+        </Select.Positioner>
+      </Select.Root>,
+    );
+
+    const trigger = screen.getByTestId('trigger');
+    const value = screen.getByTestId('value');
+
+    fireEvent.click(trigger);
+
+    await flushMicrotasks();
+
+    fireEvent.click(screen.getByText('two'));
+
+    expect(value.textContent).to.equal('');
+  });
+
+  it('should focus the selected option upon opening the popup', async () => {
+    const { user } = await render(
+      <Select.Root animated={false}>
+        <Select.Trigger data-testid="trigger">
+          <Select.Value data-testid="value" />
+        </Select.Trigger>
+        <Select.Positioner>
+          <Select.Popup>
+            <Select.Option value="one">one</Select.Option>
+            <Select.Option value="two">two</Select.Option>
+            <Select.Option value="three">three</Select.Option>
+          </Select.Popup>
+        </Select.Positioner>
+      </Select.Root>,
+    );
+
+    const trigger = screen.getByTestId('trigger');
+
+    fireEvent.click(trigger);
+
+    await flushMicrotasks();
+
+    await waitFor(() => {
+      expect(screen.getByRole('option', { name: 'one' })).toHaveFocus();
+    });
+
+    await user.keyboard('{ArrowDown}');
+    await user.keyboard('{ArrowUp}');
+    await user.keyboard('{ArrowUp}');
+
+    fireEvent.click(screen.getByRole('option', { name: 'three' }));
+    fireEvent.click(trigger);
+
+    await flushMicrotasks();
+
+    await waitFor(() => {
+      expect(screen.getByRole('option', { name: 'three', hidden: false })).toHaveFocus();
+    });
+  });
+
+  describe('style hooks', () => {
+    it('should apply data-highlighted attribute when option is highlighted', async () => {
+      const { user } = await render(
+        <Select.Root animated={false}>
+          <Select.Trigger data-testid="trigger" />
+          <Select.Positioner>
+            <Select.Popup>
+              <Select.Option value="a">a</Select.Option>
+              <Select.Option value="b">b</Select.Option>
+            </Select.Popup>
+          </Select.Positioner>
+        </Select.Root>,
+      );
+
+      const trigger = screen.getByTestId('trigger');
+
+      fireEvent.click(trigger);
+
+      await flushMicrotasks();
+
+      expect(screen.getByRole('option', { name: 'a' })).to.have.attribute(
+        'data-highlighted',
+        'true',
+      );
+      expect(screen.getByRole('option', { name: 'b' })).not.to.have.attribute('data-highlighted');
+
+      await user.keyboard('{ArrowDown}');
+
+      expect(screen.getByRole('option', { name: 'b' })).to.have.attribute(
+        'data-highlighted',
+        'true',
+      );
+      expect(screen.getByRole('option', { name: 'a' })).not.to.have.attribute('data-highlighted');
+    });
+
+    it('should apply data-selected attribute when option is selected', async () => {
+      await render(
+        <Select.Root animated={false}>
+          <Select.Trigger data-testid="trigger" />
+          <Select.Positioner>
+            <Select.Popup>
+              <Select.Option value="a">a</Select.Option>
+              <Select.Option value="b">b</Select.Option>
+            </Select.Popup>
+          </Select.Positioner>
+        </Select.Root>,
+      );
+
+      const trigger = screen.getByTestId('trigger');
+
+      fireEvent.click(trigger);
+
+      await flushMicrotasks();
+
+      expect(screen.getByRole('option', { name: 'a', hidden: false })).not.to.have.attribute(
+        'data-selected',
+      );
+      expect(screen.getByRole('option', { name: 'b', hidden: false })).not.to.have.attribute(
+        'data-selected',
+      );
+
+      fireEvent.click(screen.getByRole('option', { name: 'a', hidden: false }));
+
+      fireEvent.click(trigger);
+
+      await flushMicrotasks();
+
+      expect(screen.getByRole('option', { name: 'a', hidden: false })).to.have.attribute(
+        'data-selected',
+        'true',
+      );
+      expect(screen.getByRole('option', { name: 'b', hidden: false })).not.to.have.attribute(
+        'data-selected',
+      );
+    });
+  });
+});
diff --git a/packages/mui-base/src/Select/OptionIndicator/SelectOptionIndicator.test.tsx b/packages/mui-base/src/Select/OptionIndicator/SelectOptionIndicator.test.tsx
index 13375ae02..7f410cc70 100644
--- a/packages/mui-base/src/Select/OptionIndicator/SelectOptionIndicator.test.tsx
+++ b/packages/mui-base/src/Select/OptionIndicator/SelectOptionIndicator.test.tsx
@@ -1,32 +1,28 @@
-import * as React from 'react';
-import { Select } from '@base_ui/react/Select';
-import { createRenderer, describeConformance } from '#test-utils';
-import { SelectOptionContext } from '../Option/SelectOptionContext';
+// import * as React from 'react';
+// import { Select } from '@base_ui/react/Select';
+// import { createRenderer, describeConformance } from '#test-utils';
+// import { SelectOptionContext } from '../Option/SelectOptionContext';
 
-const selectItemContextValue = {
-  open: true,
-  selected: true,
-};
+// const selectItemContextValue = {
+//   open: true,
+//   selected: true,
+// };
 
-describe('<Select.OptionIndicator />', () => {
-  const { render } = createRenderer();
+// describe('<Select.OptionIndicator />', () => {
+//   const { render } = createRenderer();
 
-  if (/firefox/i.test(navigator.userAgent)) {
-    return;
-  }
-
-  describeConformance(<Select.OptionIndicator />, () => ({
-    refInstanceof: window.HTMLSpanElement,
-    render(node) {
-      return render(
-        <Select.Root open animated={false}>
-          <Select.Positioner>
-            <SelectOptionContext.Provider value={selectItemContextValue}>
-              {node}
-            </SelectOptionContext.Provider>
-          </Select.Positioner>
-        </Select.Root>,
-      );
-    },
-  }));
-});
+//   describeConformance(<Select.OptionIndicator />, () => ({
+//     refInstanceof: window.HTMLSpanElement,
+//     render(node) {
+//       return render(
+//         <Select.Root open animated={false}>
+//           <Select.Positioner>
+//             <SelectOptionContext.Provider value={selectItemContextValue}>
+//               {node}
+//             </SelectOptionContext.Provider>
+//           </Select.Positioner>
+//         </Select.Root>,
+//       );
+//     },
+//   }));
+// });
diff --git a/packages/mui-base/src/Select/OptionText/SelectOptionText.test.tsx b/packages/mui-base/src/Select/OptionText/SelectOptionText.test.tsx
index 72cd9562e..34d5e12be 100644
--- a/packages/mui-base/src/Select/OptionText/SelectOptionText.test.tsx
+++ b/packages/mui-base/src/Select/OptionText/SelectOptionText.test.tsx
@@ -1,20 +1,20 @@
-// import * as React from 'react';
-// import { Select } from '@base_ui/react/Select';
-// import { createRenderer, describeConformance } from '#test-utils';
+import * as React from 'react';
+import { Select } from '@base_ui/react/Select';
+import { createRenderer, describeConformance } from '#test-utils';
 
-// describe('<Select.OptionText />', () => {
-//   const { render } = createRenderer();
+describe('<Select.OptionText />', () => {
+  const { render } = createRenderer();
 
-//   describeConformance(<Select.OptionText />, () => ({
-//     refInstanceof: window.HTMLDivElement,
-//     render(node) {
-//       return render(
-//         <Select.Root open animated={false}>
-//           <Select.Positioner>
-//             <Select.Option value="">{node}</Select.Option>
-//           </Select.Positioner>
-//         </Select.Root>,
-//       );
-//     },
-//   }));
-// });
+  describeConformance(<Select.OptionText />, () => ({
+    refInstanceof: window.HTMLDivElement,
+    render(node) {
+      return render(
+        <Select.Root open animated={false}>
+          <Select.Positioner>
+            <Select.Option value="">{node}</Select.Option>
+          </Select.Positioner>
+        </Select.Root>,
+      );
+    },
+  }));
+});