diff --git a/docs/data/base/components/select/select.md b/docs/data/base/components/select/select.md
index 7b66ecc94cc074..d3c996083bff70 100644
--- a/docs/data/base/components/select/select.md
+++ b/docs/data/base/components/select/select.md
@@ -60,7 +60,7 @@ const CustomSelect = React.forwardRef(function CustomSelect<TValue>(
   return <Select {...props} ref={ref} />;
 }) as <TValue>(
   props: SelectProps<TValue> & React.RefAttributes<HTMLUListElement>,
-) => JSX.Element;
+) => React.JSX.Element;
 ```
 
 For the sake of brevity, the rest of the demos throughout this doc will not use `forwardRef`.
diff --git a/docs/data/joy/components/input/DebouncedInput.js b/docs/data/joy/components/input/DebouncedInput.js
index 7955a340238075..e3be327ae80114 100644
--- a/docs/data/joy/components/input/DebouncedInput.js
+++ b/docs/data/joy/components/input/DebouncedInput.js
@@ -7,7 +7,7 @@ import Box from '@mui/joy/Box';
 function DebounceInput(props) {
   const { handleDebounce, debounceTimeout, ...rest } = props;
 
-  const timerRef = React.useRef();
+  const timerRef = React.useRef(undefined);
 
   const handleChange = (event) => {
     clearTimeout(timerRef.current);
diff --git a/docs/data/joy/components/input/DebouncedInput.tsx b/docs/data/joy/components/input/DebouncedInput.tsx
index 927d37954daedf..5bf72ffbe9f213 100644
--- a/docs/data/joy/components/input/DebouncedInput.tsx
+++ b/docs/data/joy/components/input/DebouncedInput.tsx
@@ -11,7 +11,9 @@ type DebounceProps = {
 function DebounceInput(props: InputProps & DebounceProps) {
   const { handleDebounce, debounceTimeout, ...rest } = props;
 
-  const timerRef = React.useRef<ReturnType<typeof setTimeout>>();
+  const timerRef = React.useRef<ReturnType<typeof setTimeout> | undefined>(
+    undefined,
+  );
 
   const handleChange = (event: React.ChangeEvent<HTMLInputElement>) => {
     clearTimeout(timerRef.current);
diff --git a/docs/data/joy/components/snackbar/SnackbarHideDuration.js b/docs/data/joy/components/snackbar/SnackbarHideDuration.js
index 8c3968bd9bd7e3..6d21d8b8ec9667 100644
--- a/docs/data/joy/components/snackbar/SnackbarHideDuration.js
+++ b/docs/data/joy/components/snackbar/SnackbarHideDuration.js
@@ -10,9 +10,9 @@ export default function SnackbarHideDuration() {
   const [open, setOpen] = React.useState(false);
   const [duration, setDuration] = React.useState();
   const [left, setLeft] = React.useState();
-  const timer = React.useRef();
+  const timer = React.useRef(undefined);
   const countdown = () => {
-    timer.current = window.setInterval(() => {
+    timer.current = setInterval(() => {
       setLeft((prev) => (prev === undefined ? prev : Math.max(0, prev - 100)));
     }, 100);
   };
diff --git a/docs/data/joy/components/snackbar/SnackbarHideDuration.tsx b/docs/data/joy/components/snackbar/SnackbarHideDuration.tsx
index c98174c2cf33c5..dc56b4dfd2aed7 100644
--- a/docs/data/joy/components/snackbar/SnackbarHideDuration.tsx
+++ b/docs/data/joy/components/snackbar/SnackbarHideDuration.tsx
@@ -10,9 +10,9 @@ export default function SnackbarHideDuration() {
   const [open, setOpen] = React.useState(false);
   const [duration, setDuration] = React.useState<undefined | number>();
   const [left, setLeft] = React.useState<undefined | number>();
-  const timer = React.useRef<undefined | number>();
+  const timer = React.useRef<ReturnType<typeof setInterval> | undefined>(undefined);
   const countdown = () => {
-    timer.current = window.setInterval(() => {
+    timer.current = setInterval(() => {
       setLeft((prev) => (prev === undefined ? prev : Math.max(0, prev - 100)));
     }, 100);
   };
diff --git a/docs/data/material/components/app-bar/BackToTop.tsx b/docs/data/material/components/app-bar/BackToTop.tsx
index bb76a90931cc19..1aec6b41614cb1 100644
--- a/docs/data/material/components/app-bar/BackToTop.tsx
+++ b/docs/data/material/components/app-bar/BackToTop.tsx
@@ -16,7 +16,7 @@ interface Props {
    * You won't need it on your project.
    */
   window?: () => Window;
-  children: React.ReactElement<any>;
+  children: React.ReactElement<unknown>;
 }
 
 function ScrollTop(props: Props) {
diff --git a/docs/data/material/components/app-bar/ElevateAppBar.tsx b/docs/data/material/components/app-bar/ElevateAppBar.tsx
index cf47ff2b5cc745..aa40a9daa48303 100644
--- a/docs/data/material/components/app-bar/ElevateAppBar.tsx
+++ b/docs/data/material/components/app-bar/ElevateAppBar.tsx
@@ -13,7 +13,7 @@ interface Props {
    * You won't need it on your project.
    */
   window?: () => Window;
-  children: React.ReactElement<any>;
+  children: React.ReactElement<{ elevation?: number }>;
 }
 
 function ElevationScroll(props: Props) {
diff --git a/docs/data/material/components/app-bar/HideAppBar.tsx b/docs/data/material/components/app-bar/HideAppBar.tsx
index ee71918b61e843..cf4b6523284a2a 100644
--- a/docs/data/material/components/app-bar/HideAppBar.tsx
+++ b/docs/data/material/components/app-bar/HideAppBar.tsx
@@ -14,7 +14,7 @@ interface Props {
    * You won't need it on your project.
    */
   window?: () => Window;
-  children: React.ReactElement<any>;
+  children: React.ReactElement<unknown>;
 }
 
 function HideOnScroll(props: Props) {
diff --git a/docs/data/material/components/autocomplete/Virtualize.tsx b/docs/data/material/components/autocomplete/Virtualize.tsx
index 9e062e1ef1cec9..7a54aa5420ba91 100644
--- a/docs/data/material/components/autocomplete/Virtualize.tsx
+++ b/docs/data/material/components/autocomplete/Virtualize.tsx
@@ -58,9 +58,13 @@ const ListboxComponent = React.forwardRef<
   React.HTMLAttributes<HTMLElement>
 >(function ListboxComponent(props, ref) {
   const { children, ...other } = props;
-  const itemData: React.ReactElement<any>[] = [];
-  (children as React.ReactElement<any>[]).forEach(
-    (item: React.ReactElement<any> & { children?: React.ReactElement<any>[] }) => {
+  const itemData: React.ReactElement<unknown>[] = [];
+  (children as React.ReactElement<unknown>[]).forEach(
+    (
+      item: React.ReactElement<unknown> & {
+        children?: React.ReactElement<unknown>[];
+      },
+    ) => {
       itemData.push(item);
       itemData.push(...(item.children || []));
     },
@@ -73,7 +77,7 @@ const ListboxComponent = React.forwardRef<
   const itemCount = itemData.length;
   const itemSize = smUp ? 36 : 48;
 
-  const getChildSize = (child: React.ReactElement<any>) => {
+  const getChildSize = (child: React.ReactElement<unknown>) => {
     if (child.hasOwnProperty('group')) {
       return 48;
     }
diff --git a/docs/data/material/components/dialogs/FullScreenDialog.tsx b/docs/data/material/components/dialogs/FullScreenDialog.tsx
index 5b0dc07b9cd042..2215e96f891b12 100644
--- a/docs/data/material/components/dialogs/FullScreenDialog.tsx
+++ b/docs/data/material/components/dialogs/FullScreenDialog.tsx
@@ -15,7 +15,7 @@ import { TransitionProps } from '@mui/material/transitions';
 
 const Transition = React.forwardRef(function Transition(
   props: TransitionProps & {
-    children: React.ReactElement<any>;
+    children: React.ReactElement<unknown>;
   },
   ref: React.Ref<unknown>,
 ) {
diff --git a/docs/data/material/components/lists/InteractiveList.tsx b/docs/data/material/components/lists/InteractiveList.tsx
index 661995d60917d4..e667a977cda23a 100644
--- a/docs/data/material/components/lists/InteractiveList.tsx
+++ b/docs/data/material/components/lists/InteractiveList.tsx
@@ -16,7 +16,7 @@ import Typography from '@mui/material/Typography';
 import FolderIcon from '@mui/icons-material/Folder';
 import DeleteIcon from '@mui/icons-material/Delete';
 
-function generate(element: React.ReactElement<any>) {
+function generate(element: React.ReactElement<unknown>) {
   return [0, 1, 2].map((value) =>
     React.cloneElement(element, {
       key: value,
diff --git a/docs/data/material/components/popper/SpringPopper.tsx b/docs/data/material/components/popper/SpringPopper.tsx
index 5526317503ff16..636bedcfc229c1 100644
--- a/docs/data/material/components/popper/SpringPopper.tsx
+++ b/docs/data/material/components/popper/SpringPopper.tsx
@@ -4,7 +4,7 @@ import Popper from '@mui/material/Popper';
 import { useSpring, animated } from '@react-spring/web';
 
 interface FadeProps {
-  children?: React.ReactElement<any>;
+  children?: React.ReactElement<unknown>;
   in?: boolean;
   onEnter?: () => void;
   onExited?: () => void;
diff --git a/docs/data/material/components/progress/CircularIntegration.js b/docs/data/material/components/progress/CircularIntegration.js
index 1c885b355996da..adb2accfe4cd9a 100644
--- a/docs/data/material/components/progress/CircularIntegration.js
+++ b/docs/data/material/components/progress/CircularIntegration.js
@@ -10,7 +10,7 @@ import SaveIcon from '@mui/icons-material/Save';
 export default function CircularIntegration() {
   const [loading, setLoading] = React.useState(false);
   const [success, setSuccess] = React.useState(false);
-  const timer = React.useRef();
+  const timer = React.useRef(undefined);
 
   const buttonSx = {
     ...(success && {
diff --git a/docs/data/material/components/progress/CircularIntegration.tsx b/docs/data/material/components/progress/CircularIntegration.tsx
index c32a5957a42fb8..c0e3761c48905b 100644
--- a/docs/data/material/components/progress/CircularIntegration.tsx
+++ b/docs/data/material/components/progress/CircularIntegration.tsx
@@ -10,7 +10,7 @@ import SaveIcon from '@mui/icons-material/Save';
 export default function CircularIntegration() {
   const [loading, setLoading] = React.useState(false);
   const [success, setSuccess] = React.useState(false);
-  const timer = React.useRef<ReturnType<typeof setTimeout>>();
+  const timer = React.useRef<ReturnType<typeof setTimeout> | undefined>(undefined);
 
   const buttonSx = {
     ...(success && {
diff --git a/docs/data/material/components/progress/DelayingAppearance.js b/docs/data/material/components/progress/DelayingAppearance.js
index df2b050772e529..0844371d452fbf 100644
--- a/docs/data/material/components/progress/DelayingAppearance.js
+++ b/docs/data/material/components/progress/DelayingAppearance.js
@@ -8,7 +8,7 @@ import Typography from '@mui/material/Typography';
 export default function DelayingAppearance() {
   const [loading, setLoading] = React.useState(false);
   const [query, setQuery] = React.useState('idle');
-  const timerRef = React.useRef();
+  const timerRef = React.useRef(undefined);
 
   React.useEffect(
     () => () => {
diff --git a/docs/data/material/components/progress/DelayingAppearance.tsx b/docs/data/material/components/progress/DelayingAppearance.tsx
index a6784fd2d359d8..024769f55ca7a0 100644
--- a/docs/data/material/components/progress/DelayingAppearance.tsx
+++ b/docs/data/material/components/progress/DelayingAppearance.tsx
@@ -8,7 +8,9 @@ import Typography from '@mui/material/Typography';
 export default function DelayingAppearance() {
   const [loading, setLoading] = React.useState(false);
   const [query, setQuery] = React.useState('idle');
-  const timerRef = React.useRef<ReturnType<typeof setTimeout>>();
+  const timerRef = React.useRef<ReturnType<typeof setTimeout> | undefined>(
+    undefined,
+  );
 
   React.useEffect(
     () => () => {
diff --git a/docs/data/material/components/rating/RadioGroupRating.tsx b/docs/data/material/components/rating/RadioGroupRating.tsx
index f42709d21c1359..89a509e2d0effe 100644
--- a/docs/data/material/components/rating/RadioGroupRating.tsx
+++ b/docs/data/material/components/rating/RadioGroupRating.tsx
@@ -15,7 +15,7 @@ const StyledRating = styled(Rating)(({ theme }) => ({
 
 const customIcons: {
   [index: string]: {
-    icon: React.ReactElement<any>;
+    icon: React.ReactElement<unknown>;
     label: string;
   };
 } = {
diff --git a/docs/data/material/components/steppers/CustomizedSteppers.tsx b/docs/data/material/components/steppers/CustomizedSteppers.tsx
index 3a4c2e7ddf879c..03791f59e854a9 100644
--- a/docs/data/material/components/steppers/CustomizedSteppers.tsx
+++ b/docs/data/material/components/steppers/CustomizedSteppers.tsx
@@ -122,7 +122,7 @@ const ColorlibStepIconRoot = styled('div')<{
 function ColorlibStepIcon(props: StepIconProps) {
   const { active, completed, className } = props;
 
-  const icons: { [index: string]: React.ReactElement<any> } = {
+  const icons: { [index: string]: React.ReactElement<unknown> } = {
     1: <SettingsIcon />,
     2: <GroupAddIcon />,
     3: <VideoLabelIcon />,
diff --git a/docs/data/material/integrations/routing/ListRouter.tsx b/docs/data/material/integrations/routing/ListRouter.tsx
index fc3fe2410991a3..52dac8f8461185 100644
--- a/docs/data/material/integrations/routing/ListRouter.tsx
+++ b/docs/data/material/integrations/routing/ListRouter.tsx
@@ -34,7 +34,7 @@ function Router(props: { children?: React.ReactNode }) {
 }
 
 interface ListItemLinkProps {
-  icon?: React.ReactElement<any>;
+  icon?: React.ReactElement<unknown>;
   primary: string;
   to: string;
 }
diff --git a/docs/pages/base-ui/api/use-autocomplete.json b/docs/pages/base-ui/api/use-autocomplete.json
index 994ebed0543af0..0a4ac4aaf008aa 100644
--- a/docs/pages/base-ui/api/use-autocomplete.json
+++ b/docs/pages/base-ui/api/use-autocomplete.json
@@ -147,8 +147,8 @@
     },
     "unstable_isActiveElementInListbox": {
       "type": {
-        "name": "(listbox: React.RefObject&lt;HTMLElement&gt;) =&gt; boolean",
-        "description": "(listbox: React.RefObject&lt;HTMLElement&gt;) =&gt; boolean"
+        "name": "(listbox: React.RefObject&lt;HTMLElement | null&gt;) =&gt; boolean",
+        "description": "(listbox: React.RefObject&lt;HTMLElement | null&gt;) =&gt; boolean"
       }
     },
     "value": {
diff --git a/docs/src/components/about/HowToSupport.tsx b/docs/src/components/about/HowToSupport.tsx
index bf03040aab35a2..2ac0a6e22aaa24 100644
--- a/docs/src/components/about/HowToSupport.tsx
+++ b/docs/src/components/about/HowToSupport.tsx
@@ -21,7 +21,7 @@ function Widget({
 }: {
   children: React.ReactNode;
   title: string;
-  icon: React.ReactElement<any>;
+  icon: React.ReactElement<unknown>;
 }) {
   return (
     <Paper
diff --git a/docs/src/components/header/HeaderNavBar.tsx b/docs/src/components/header/HeaderNavBar.tsx
index f135aa52fb9138..8fe75798908401 100644
--- a/docs/src/components/header/HeaderNavBar.tsx
+++ b/docs/src/components/header/HeaderNavBar.tsx
@@ -75,7 +75,7 @@ const PRODUCT_IDS = [
 ];
 
 type ProductSubMenuProps = {
-  icon: React.ReactElement<any>;
+  icon: React.ReactElement<unknown>;
   name: React.ReactNode;
   description: React.ReactNode;
   chip?: React.ReactNode;
diff --git a/docs/src/components/home/MaterialDesignComponents.tsx b/docs/src/components/home/MaterialDesignComponents.tsx
index 645d2786cc7410..efacc3c318452d 100644
--- a/docs/src/components/home/MaterialDesignComponents.tsx
+++ b/docs/src/components/home/MaterialDesignComponents.tsx
@@ -125,7 +125,7 @@ function Demo({
 }: {
   name: string;
   theme: Theme | undefined;
-  children: React.ReactElement<any>;
+  children: React.ReactElement<unknown>;
   control?: { prop: string; values: Array<string>; defaultValue?: string };
 }) {
   const [propValue, setPropValue] = React.useState(
diff --git a/docs/src/components/home/UserFeedbacks.tsx b/docs/src/components/home/UserFeedbacks.tsx
index 5f9786f3d08b66..5a73469ffab2f7 100644
--- a/docs/src/components/home/UserFeedbacks.tsx
+++ b/docs/src/components/home/UserFeedbacks.tsx
@@ -95,7 +95,7 @@ function Feedback({
     avatarSrcSet: string;
     name: string;
     role: string;
-    company?: React.ReactElement<any>;
+    company?: React.ReactElement<unknown>;
   };
 }) {
   return (
diff --git a/docs/src/components/pricing/PricingTable.tsx b/docs/src/components/pricing/PricingTable.tsx
index e25e774399bac0..d8c225ab266adc 100644
--- a/docs/src/components/pricing/PricingTable.tsx
+++ b/docs/src/components/pricing/PricingTable.tsx
@@ -424,7 +424,7 @@ function RowHead({
   children,
   startIcon,
   ...props
-}: BoxProps & { startIcon?: React.ReactElement<any> }) {
+}: BoxProps & { startIcon?: React.ReactElement<unknown> }) {
   return (
     <Box
       {...props}
diff --git a/docs/src/components/productX/XRoadmap.tsx b/docs/src/components/productX/XRoadmap.tsx
index 8eb2b64f83b924..e43b55bd5a67b3 100644
--- a/docs/src/components/productX/XRoadmap.tsx
+++ b/docs/src/components/productX/XRoadmap.tsx
@@ -41,7 +41,7 @@ function RoadmapStatusDot({ color }: RoadmapStatusDotProps) {
 }
 
 export default function XRoadmap() {
-  function renderList(content: React.ReactElement<any>, nested?: boolean) {
+  function renderList(content: React.ReactElement<unknown>, nested?: boolean) {
     return (
       <Box
         sx={{
diff --git a/docs/src/components/showcase/ViewToggleButton.tsx b/docs/src/components/showcase/ViewToggleButton.tsx
index e4c155cbd9c987..eec9402524ea2c 100644
--- a/docs/src/components/showcase/ViewToggleButton.tsx
+++ b/docs/src/components/showcase/ViewToggleButton.tsx
@@ -11,7 +11,7 @@ const views = ['quilt', 'module', 'agenda', 'week', 'sidebar'] as const;
 
 type View = (typeof views)[number];
 
-const viewIcons: Record<View, React.ReactElement<any>> = {
+const viewIcons: Record<View, React.ReactElement<unknown>> = {
   quilt: <ViewQuiltRounded />,
   module: <ViewModuleRounded />,
   agenda: <ViewAgendaRounded />,
diff --git a/docs/src/components/typography/SectionHeadline.tsx b/docs/src/components/typography/SectionHeadline.tsx
index b1e30e3eb76611..f9d42e1659d68e 100644
--- a/docs/src/components/typography/SectionHeadline.tsx
+++ b/docs/src/components/typography/SectionHeadline.tsx
@@ -11,7 +11,7 @@ interface SectionHeadlineProps {
    */
   inverted?: boolean;
   overline?: React.ReactNode;
-  title: string | React.ReactElement<any>;
+  title: string | React.ReactElement<React.HTMLAttributes<HTMLElement>>;
 }
 
 export default function SectionHeadline(props: SectionHeadlineProps) {
diff --git a/docs/src/layouts/HeroContainer.tsx b/docs/src/layouts/HeroContainer.tsx
index 0fcd853ab1a7b9..536dc4f17fd392 100644
--- a/docs/src/layouts/HeroContainer.tsx
+++ b/docs/src/layouts/HeroContainer.tsx
@@ -8,9 +8,9 @@ import { alpha } from '@mui/material/styles';
 interface HeroContainerProps {
   disableMobileHidden?: boolean;
   disableTabExclusion?: boolean;
-  left: React.ReactElement<any>;
+  left: React.ReactElement<unknown>;
   linearGradient?: boolean;
-  right: React.ReactElement<any>;
+  right: React.ReactElement<unknown>;
   rightSx?: BoxProps['sx'];
 }
 
diff --git a/docs/src/modules/components/JoyThemeBuilder.tsx b/docs/src/modules/components/JoyThemeBuilder.tsx
index e8507ace5d0671..902745b23081ae 100644
--- a/docs/src/modules/components/JoyThemeBuilder.tsx
+++ b/docs/src/modules/components/JoyThemeBuilder.tsx
@@ -983,7 +983,11 @@ function filterGlobalVariantTokens(palette: Partial<PaletteVariant>, variant: Va
   return tokens;
 }
 
-type StateReducer<T> = (state: T, action: Partial<T>) => T;
+type ReducerState = {
+  hover: boolean;
+  active: boolean;
+  disabled: boolean;
+};
 
 function GlobalVariantForm({
   color,
@@ -1001,13 +1005,14 @@ function GlobalVariantForm({
   onRemove: (token: string) => void;
 }) {
   const [selectedVariant, setSelectedVariant] = React.useState<VariantProp>('solid');
-  const [states, setStates] = React.useReducer<
-    StateReducer<{ hover: boolean; active: boolean; disabled: boolean }>
-  >((prevState, action) => ({ ...prevState, ...action }), {
-    hover: false,
-    active: false,
-    disabled: false,
-  });
+  const [states, setStates] = React.useReducer(
+    (prevState: ReducerState, action: Partial<ReducerState>) => ({ ...prevState, ...action }),
+    {
+      hover: false,
+      active: false,
+      disabled: false,
+    },
+  );
   const themeDefaultValue = filterGlobalVariantTokens(themeDefaultValueProp, selectedVariant);
   const value = filterGlobalVariantTokens(valueProp, selectedVariant);
   const mergedValue = { ...themeDefaultValue, ...value };
diff --git a/docs/src/modules/components/JoyUsageDemo.tsx b/docs/src/modules/components/JoyUsageDemo.tsx
index ea9b394509fda9..7da2e3bb8b7197 100644
--- a/docs/src/modules/components/JoyUsageDemo.tsx
+++ b/docs/src/modules/components/JoyUsageDemo.tsx
@@ -165,7 +165,7 @@ interface JoyUsageDemoProps<ComponentProps> {
    * A function to override the code block result.
    */
   getCodeBlock?: (code: string, props: ComponentProps) => string;
-  renderDemo: (props: ComponentProps) => React.ReactElement<any>;
+  renderDemo: (props: ComponentProps) => React.ReactElement<unknown>;
 }
 
 export default function JoyUsageDemo<T extends { [k: string]: any } = {}>({
diff --git a/docs/src/modules/components/JoyVariablesDemo.tsx b/docs/src/modules/components/JoyVariablesDemo.tsx
index 16450717fbe363..b23e4bf952bee3 100644
--- a/docs/src/modules/components/JoyVariablesDemo.tsx
+++ b/docs/src/modules/components/JoyVariablesDemo.tsx
@@ -33,7 +33,7 @@ function formatSx(sx: { [k: string]: string | number }) {
 interface SlotVariablesProps {
   slot: string;
   data: Array<DataItem>;
-  renderField: (item: DataItem) => React.ReactElement<any>;
+  renderField: (item: DataItem) => React.ReactElement<unknown>;
   defaultOpen?: boolean;
 }
 
@@ -88,7 +88,7 @@ export default function JoyVariablesDemo(props: {
   componentName: string;
   childrenAccepted?: boolean;
   data: Array<DataItem | [string, Array<DataItem>, { defaultOpen?: boolean } | undefined]>;
-  renderDemo: (sx: { [k: string]: string | number }) => React.ReactElement<any>;
+  renderDemo: (sx: { [k: string]: string | number }) => React.ReactElement<unknown>;
   renderCode?: (formattedSx: string) => string;
 }) {
   const { componentName, data = [], childrenAccepted = false, renderCode } = props;
diff --git a/docs/types/docs.d.ts b/docs/types/docs.d.ts
index fc96dea50c386a..4d77ba0ecabb66 100644
--- a/docs/types/docs.d.ts
+++ b/docs/types/docs.d.ts
@@ -23,7 +23,7 @@ declare module 'docs/src/modules/components/HighlightedCode' {
     component?: React.ElementType;
     sx?: object;
   }
-  export default function HighlightedCode(props: Props): React.ReactElement<any>;
+  export default function HighlightedCode(props: Props): React.ReactElement<unknown>;
 }
 
 declare module 'react-imask';
diff --git a/packages/mui-base/src/Button/Button.tsx b/packages/mui-base/src/Button/Button.tsx
index 816c3e9c4f1c71..ce9dd0ce7fa9cb 100644
--- a/packages/mui-base/src/Button/Button.tsx
+++ b/packages/mui-base/src/Button/Button.tsx
@@ -46,7 +46,7 @@ const Button = React.forwardRef(function Button<RootComponentType extends React.
     ...other
   } = props;
 
-  const buttonRef = React.useRef<HTMLButtonElement | HTMLAnchorElement | HTMLElement>();
+  const buttonRef = React.useRef<HTMLButtonElement | HTMLAnchorElement | HTMLElement | null>(null);
 
   let rootElementName = rootElementNameProp;
 
diff --git a/packages/mui-base/src/Tab/Tab.tsx b/packages/mui-base/src/Tab/Tab.tsx
index d63b03374d05bd..6147ba91e90ed3 100644
--- a/packages/mui-base/src/Tab/Tab.tsx
+++ b/packages/mui-base/src/Tab/Tab.tsx
@@ -45,7 +45,7 @@ const Tab = React.forwardRef(function Tab<RootComponentType extends React.Elemen
     ...other
   } = props;
 
-  const tabRef = React.useRef<HTMLButtonElement | HTMLAnchorElement | HTMLElement>();
+  const tabRef = React.useRef<HTMLButtonElement | HTMLAnchorElement | HTMLElement | null>(null);
   const handleRef = useForkRef(tabRef, forwardedRef);
 
   const { active, highlighted, selected, getRootProps } = useTab({
diff --git a/packages/mui-base/src/useAutocomplete/useAutocomplete.d.ts b/packages/mui-base/src/useAutocomplete/useAutocomplete.d.ts
index f436583759b0bf..d2825047f2c0c7 100644
--- a/packages/mui-base/src/useAutocomplete/useAutocomplete.d.ts
+++ b/packages/mui-base/src/useAutocomplete/useAutocomplete.d.ts
@@ -49,7 +49,7 @@ export interface UseAutocompleteProps<
    * Temporary for Joy UI because the parent listbox is the document object
    * TODO v6: Normalize the logic and remove this param.
    */
-  unstable_isActiveElementInListbox?: (listbox: React.RefObject<HTMLElement>) => boolean;
+  unstable_isActiveElementInListbox?: (listbox: React.RefObject<HTMLElement | null>) => boolean;
   /**
    * If `true`, the portion of the selected suggestion that the user hasn't typed,
    * known as the completion string, appears inline after the input cursor in the textbox.
diff --git a/packages/mui-base/src/useButton/useButton.ts b/packages/mui-base/src/useButton/useButton.ts
index 0c6b5da72973aa..815a3009b2fa70 100644
--- a/packages/mui-base/src/useButton/useButton.ts
+++ b/packages/mui-base/src/useButton/useButton.ts
@@ -34,7 +34,7 @@ export function useButton(parameters: UseButtonParameters = {}): UseButtonReturn
     type,
     rootElementName: rootElementNameProp,
   } = parameters;
-  const buttonRef = React.useRef<HTMLButtonElement | HTMLAnchorElement | HTMLElement>();
+  const buttonRef = React.useRef<HTMLButtonElement | HTMLAnchorElement | HTMLElement | null>(null);
 
   const [active, setActive] = React.useState<boolean>(false);
 
diff --git a/packages/mui-base/src/useCompound/useCompound.test.tsx b/packages/mui-base/src/useCompound/useCompound.test.tsx
index 509e851cf6456b..4e51d94ac76afe 100644
--- a/packages/mui-base/src/useCompound/useCompound.test.tsx
+++ b/packages/mui-base/src/useCompound/useCompound.test.tsx
@@ -4,7 +4,7 @@ import { render } from '@mui-internal/test-utils';
 import { CompoundComponentContext, useCompoundParent } from './useCompoundParent';
 import { useCompoundItem } from './useCompoundItem';
 
-type ItemValue = { value: string; ref: React.RefObject<HTMLSpanElement> };
+type ItemValue = { value: string; ref: React.RefObject<HTMLSpanElement | null> };
 
 describe('compound components', () => {
   describe('useCompoundParent', () => {
@@ -188,7 +188,7 @@ describe('compound components', () => {
         const { children } = props;
         const { contextValue } = useCompoundParent<
           string,
-          { ref: React.RefObject<HTMLSpanElement> }
+          { ref: React.RefObject<HTMLSpanElement | null> }
         >();
 
         return (
@@ -240,7 +240,7 @@ describe('compound components', () => {
         const { children } = props;
         const { contextValue } = useCompoundParent<
           number,
-          { ref: React.RefObject<HTMLLIElement> }
+          { ref: React.RefObject<HTMLLIElement | null> }
         >();
 
         return (
@@ -256,7 +256,7 @@ describe('compound components', () => {
 
       function Child() {
         const ref = React.useRef<HTMLLIElement>(null);
-        const { id } = useCompoundItem<string, { ref: React.RefObject<HTMLLIElement> }>(
+        const { id } = useCompoundItem<string, { ref: React.RefObject<HTMLLIElement | null> }>(
           idGenerator,
           React.useMemo(() => ({ ref }), []),
         );
diff --git a/packages/mui-base/src/useCompound/useCompoundParent.ts b/packages/mui-base/src/useCompound/useCompoundParent.ts
index 8cc2dad24b6efe..99f0b2082704ff 100644
--- a/packages/mui-base/src/useCompound/useCompoundParent.ts
+++ b/packages/mui-base/src/useCompound/useCompoundParent.ts
@@ -47,7 +47,10 @@ if (process.env.NODE_ENV !== 'production') {
   CompoundComponentContext.displayName = 'CompoundComponentContext';
 }
 
-export interface UseCompoundParentReturnValue<Key, Subitem extends { ref: React.RefObject<Node> }> {
+export interface UseCompoundParentReturnValue<
+  Key,
+  Subitem extends { ref: React.RefObject<Node | null> },
+> {
   /**
    * The value for the CompoundComponentContext provider.
    */
@@ -63,7 +66,7 @@ export interface UseCompoundParentReturnValue<Key, Subitem extends { ref: React.
 /**
  * Sorts the subitems by their position in the DOM.
  */
-function sortSubitems<Key, Subitem extends { ref: React.RefObject<Node> }>(
+function sortSubitems<Key, Subitem extends { ref: React.RefObject<Node | null> }>(
   subitems: Map<Key, Subitem>,
 ) {
   const subitemsArray = Array.from(subitems.keys()).map((key) => {
@@ -100,7 +103,7 @@ function sortSubitems<Key, Subitem extends { ref: React.RefObject<Node> }>(
  */
 export function useCompoundParent<
   Key,
-  Subitem extends { ref: React.RefObject<Node> },
+  Subitem extends { ref: React.RefObject<Node | null> },
 >(): UseCompoundParentReturnValue<Key, Subitem> {
   const [subitems, setSubitems] = React.useState(new Map<Key, Subitem>());
   const subitemKeys = React.useRef(new Set<Key>());
diff --git a/packages/mui-base/src/useMenu/menuReducer.ts b/packages/mui-base/src/useMenu/menuReducer.ts
index 74ffc00361bbe0..ecdf640b4fe16f 100644
--- a/packages/mui-base/src/useMenu/menuReducer.ts
+++ b/packages/mui-base/src/useMenu/menuReducer.ts
@@ -3,7 +3,7 @@ import { ActionWithContext } from '../utils/useControllableReducer.types';
 import { MenuInternalState } from './useMenu.types';
 
 export type MenuActionContext = ListActionContext<string> & {
-  listboxRef: React.RefObject<HTMLElement>;
+  listboxRef: React.RefObject<HTMLElement | null>;
 };
 
 export function menuReducer(
diff --git a/packages/mui-base/src/useMenuItem/useMenuItem.types.ts b/packages/mui-base/src/useMenuItem/useMenuItem.types.ts
index 8f5231685ec892..ed8ea501838a62 100644
--- a/packages/mui-base/src/useMenuItem/useMenuItem.types.ts
+++ b/packages/mui-base/src/useMenuItem/useMenuItem.types.ts
@@ -11,7 +11,7 @@ export interface MenuItemMetadata {
   id: string;
   disabled: boolean;
   label?: string;
-  ref: React.RefObject<HTMLElement>;
+  ref: React.RefObject<HTMLElement | null>;
 }
 
 export type UseMenuItemRootSlotProps<ExternalProps = {}> = ExternalProps &
diff --git a/packages/mui-base/src/useOption/useOption.types.ts b/packages/mui-base/src/useOption/useOption.types.ts
index 0bc3ee4cf14d23..e64f59bef52229 100644
--- a/packages/mui-base/src/useOption/useOption.types.ts
+++ b/packages/mui-base/src/useOption/useOption.types.ts
@@ -4,7 +4,7 @@ export interface SelectOption<Value> {
   value: Value;
   label: React.ReactNode;
   disabled?: boolean;
-  ref: React.RefObject<HTMLElement>;
+  ref: React.RefObject<HTMLElement | null>;
   id?: string;
 }
 
diff --git a/packages/mui-base/src/useSlider/useSlider.ts b/packages/mui-base/src/useSlider/useSlider.ts
index 7739bab422238c..564a1f3612627a 100644
--- a/packages/mui-base/src/useSlider/useSlider.ts
+++ b/packages/mui-base/src/useSlider/useSlider.ts
@@ -218,7 +218,7 @@ export function useSlider(parameters: UseSliderParameters): UseSliderReturnValue
     value: valueProp,
   } = parameters;
 
-  const touchId = React.useRef<number>();
+  const touchId = React.useRef<number | undefined>(undefined);
   // We can't use the :active browser pseudo-classes.
   // - The active state isn't triggered when clicking on the rail.
   // - The active state isn't transferred when inversing a range slider.
@@ -273,7 +273,7 @@ export function useSlider(parameters: UseSliderParameters): UseSliderReturnValue
   } = useIsFocusVisible();
   const [focusedThumbIndex, setFocusedThumbIndex] = React.useState(-1);
 
-  const sliderRef = React.useRef<HTMLSpanElement>();
+  const sliderRef = React.useRef<HTMLSpanElement | null>(null);
   const handleFocusRef = useForkRef(focusVisibleRef, sliderRef);
   const handleRef = useForkRef(ref, handleFocusRef);
 
@@ -405,7 +405,7 @@ export function useSlider(parameters: UseSliderParameters): UseSliderReturnValue
       changeValue(event, event.target.valueAsNumber);
     };
 
-  const previousIndex = React.useRef<number>();
+  const previousIndex = React.useRef<number | undefined>(undefined);
   let axis = orientation;
   if (isRtl && orientation === 'horizontal') {
     axis += '-reverse';
diff --git a/packages/mui-base/src/useTabs/TabsProvider.tsx b/packages/mui-base/src/useTabs/TabsProvider.tsx
index 31d8aebd939315..2731205d1a8511 100644
--- a/packages/mui-base/src/useTabs/TabsProvider.tsx
+++ b/packages/mui-base/src/useTabs/TabsProvider.tsx
@@ -5,7 +5,7 @@ import { CompoundComponentContext, CompoundComponentContextValue } from '../useC
 
 export type TabPanelMetadata = {
   id: string | undefined;
-  ref: React.RefObject<HTMLElement>;
+  ref: React.RefObject<HTMLElement | null>;
 };
 
 export type TabsProviderValue = CompoundComponentContextValue<string | number, TabPanelMetadata> &
diff --git a/packages/mui-base/src/useTabs/useTabs.ts b/packages/mui-base/src/useTabs/useTabs.ts
index 75994646e1266c..bf6e1d368f8f70 100644
--- a/packages/mui-base/src/useTabs/useTabs.ts
+++ b/packages/mui-base/src/useTabs/useTabs.ts
@@ -8,7 +8,7 @@ import { TabPanelMetadata } from './TabsProvider';
 export interface TabMetadata {
   disabled: boolean;
   id: string | undefined;
-  ref: React.RefObject<HTMLElement>;
+  ref: React.RefObject<HTMLElement | null>;
 }
 
 type IdLookupFunction = (id: string | number) => string | undefined;
diff --git a/packages/mui-base/src/utils/useMessageBus.ts b/packages/mui-base/src/utils/useMessageBus.ts
index b7246f9b0b4bce..4cfab271580c1f 100644
--- a/packages/mui-base/src/utils/useMessageBus.ts
+++ b/packages/mui-base/src/utils/useMessageBus.ts
@@ -40,7 +40,7 @@ export function createMessageBus(): MessageBus {
  * @ignore - internal hook.
  */
 export function useMessageBus() {
-  const bus = React.useRef<MessageBus>();
+  const bus = React.useRef<MessageBus | undefined>(undefined);
   if (!bus.current) {
     bus.current = createMessageBus();
   }
diff --git a/packages/mui-base/test/describeConformanceUnstyled.tsx b/packages/mui-base/test/describeConformanceUnstyled.tsx
index 47efb7c54f4bdd..516ea6d92d757f 100644
--- a/packages/mui-base/test/describeConformanceUnstyled.tsx
+++ b/packages/mui-base/test/describeConformanceUnstyled.tsx
@@ -267,7 +267,7 @@ function testSlotPropsProp(
   });
 }
 
-function testClassName(element: React.ReactElement, getOptions: () => ConformanceOptions) {
+function testClassName(element: React.ReactElement<any>, getOptions: () => ConformanceOptions) {
   it('applies the className to the root component', async () => {
     const { render } = getOptions();
 
diff --git a/packages/mui-joy/src/Autocomplete/Autocomplete.tsx b/packages/mui-joy/src/Autocomplete/Autocomplete.tsx
index 96bbf5621d67ef..1d688d448de91e 100644
--- a/packages/mui-joy/src/Autocomplete/Autocomplete.tsx
+++ b/packages/mui-joy/src/Autocomplete/Autocomplete.tsx
@@ -45,7 +45,7 @@ import useSlot from '../utils/useSlot';
 
 type OwnerState = Omit<AutocompleteOwnerState<any, any, any, any>, 'onChange' | 'defaultValue'>;
 
-const defaultIsActiveElementInListbox = (listboxRef: React.RefObject<HTMLElement>) =>
+const defaultIsActiveElementInListbox = (listboxRef: React.RefObject<HTMLElement | null>) =>
   listboxRef.current !== null && listboxRef.current.contains(document.activeElement);
 // @ts-ignore
 const defaultGetOptionLabel = (option) => option.label ?? option;
diff --git a/packages/mui-joy/src/ButtonGroup/ButtonGroup.tsx b/packages/mui-joy/src/ButtonGroup/ButtonGroup.tsx
index 00cd3fbff6f73c..5e4dfb829c243c 100644
--- a/packages/mui-joy/src/ButtonGroup/ButtonGroup.tsx
+++ b/packages/mui-joy/src/ButtonGroup/ButtonGroup.tsx
@@ -18,6 +18,7 @@ import ButtonGroupContext from './ButtonGroupContext';
 import useSlot from '../utils/useSlot';
 import buttonClasses from '../Button/buttonClasses';
 import iconButtonClasses from '../IconButton/iconButtonClasses';
+import { DividerProps } from '../Divider';
 
 const useUtilityClasses = (ownerState: ButtonGroupOwnerState) => {
   const { size, variant, color, orientation } = ownerState;
@@ -236,11 +237,11 @@ const ButtonGroup = React.forwardRef(function ButtonGroup(inProps, ref) {
           }
           const extraProps: Record<string, any> = {};
           if (isMuiElement(child, ['Divider'])) {
-            extraProps.inset = 'inset' in child.props ? child.props.inset : 'context';
+            const childProps = child.props as DividerProps;
+            extraProps.inset = childProps?.inset ?? 'context';
 
             const dividerOrientation = orientation === 'vertical' ? 'horizontal' : 'vertical';
-            extraProps.orientation =
-              'orientation' in child.props ? child.props.orientation : dividerOrientation;
+            extraProps.orientation = childProps?.orientation ?? dividerOrientation;
             extraProps.role = 'presentation';
             extraProps.component = 'span';
           }
diff --git a/packages/mui-joy/src/Card/Card.tsx b/packages/mui-joy/src/Card/Card.tsx
index 3fdd38efa29774..7ee7eda5de52d1 100644
--- a/packages/mui-joy/src/Card/Card.tsx
+++ b/packages/mui-joy/src/Card/Card.tsx
@@ -15,6 +15,7 @@ import { getCardUtilityClass } from './cardClasses';
 import { CardProps, CardOwnerState, CardTypeMap } from './CardProps';
 import { resolveSxValue } from '../styles/styleUtils';
 import useSlot from '../utils/useSlot';
+import { DividerProps } from '../Divider';
 
 const useUtilityClasses = (ownerState: CardOwnerState) => {
   const { size, variant, color, orientation } = ownerState;
@@ -166,11 +167,11 @@ const Card = React.forwardRef(function Card(inProps, ref) {
         }
         const extraProps: Record<string, any> = {};
         if (isMuiElement(child, ['Divider'])) {
-          extraProps.inset = 'inset' in child.props ? child.props.inset : 'context';
+          const childProps = child.props as DividerProps;
+          extraProps.inset = childProps?.inset ?? 'context';
 
           const dividerOrientation = orientation === 'vertical' ? 'horizontal' : 'vertical';
-          extraProps.orientation =
-            'orientation' in child.props ? child.props.orientation : dividerOrientation;
+          extraProps.orientation = childProps?.orientation ?? dividerOrientation;
         }
         if (index === 0) {
           extraProps['data-first-child'] = '';
diff --git a/packages/mui-joy/src/ListItem/ListItem.tsx b/packages/mui-joy/src/ListItem/ListItem.tsx
index 3e9eb9baba8cc0..00c73d6a801214 100644
--- a/packages/mui-joy/src/ListItem/ListItem.tsx
+++ b/packages/mui-joy/src/ListItem/ListItem.tsx
@@ -11,7 +11,7 @@ import { unstable_composeClasses as composeClasses } from '@mui/base/composeClas
 import { styled, useThemeProps } from '../styles';
 
 import useSlot from '../utils/useSlot';
-import { ListItemOwnerState, ListItemTypeMap } from './ListItemProps';
+import { ListItemOwnerState, ListItemProps, ListItemTypeMap } from './ListItemProps';
 import listItemClasses, { getListItemUtilityClass } from './listItemClasses';
 import NestedListContext from '../List/NestedListContext';
 import RowListContext from '../List/RowListContext';
@@ -256,7 +256,7 @@ const ListItem = React.forwardRef(function ListItem(inProps, ref) {
                   ...(index === 0 && { 'data-first-child': '' }),
                   ...(isMuiElement(child, ['ListItem']) && {
                     // The ListItem of ListItem should not be 'li'
-                    component: child.props.component || 'div',
+                    component: (child.props as ListItemProps)?.component || 'div',
                   }),
                 })
               : child,
diff --git a/packages/mui-joy/src/ModalDialog/ModalDialog.tsx b/packages/mui-joy/src/ModalDialog/ModalDialog.tsx
index 36c6da2fd58d8d..633ee0ee3b2d2f 100644
--- a/packages/mui-joy/src/ModalDialog/ModalDialog.tsx
+++ b/packages/mui-joy/src/ModalDialog/ModalDialog.tsx
@@ -18,6 +18,7 @@ import ModalDialogSizeContext from './ModalDialogSizeContext';
 import ModalDialogVariantColorContext from './ModalDialogVariantColorContext';
 import useSlot from '../utils/useSlot';
 import { StyledCardRoot } from '../Card/Card';
+import { DividerProps } from '../Divider';
 
 const useUtilityClasses = (ownerState: ModalDialogOwnerState) => {
   const { variant, color, size, layout } = ownerState;
@@ -195,11 +196,11 @@ const ModalDialog = React.forwardRef(function ModalDialog(inProps, ref) {
             }
             const extraProps: Record<string, any> = {};
             if (isMuiElement(child, ['Divider'])) {
-              extraProps.inset = 'inset' in child.props ? child.props.inset : 'context';
+              const childProps = child.props as DividerProps;
+              extraProps.inset = childProps?.inset ?? 'context';
 
               const dividerOrientation = orientation === 'vertical' ? 'horizontal' : 'vertical';
-              extraProps.orientation =
-                'orientation' in child.props ? child.props.orientation : dividerOrientation;
+              extraProps.orientation = childProps?.orientation ?? dividerOrientation;
             }
             if (index === 0) {
               extraProps['data-first-child'] = '';
diff --git a/packages/mui-joy/src/Tab/Tab.tsx b/packages/mui-joy/src/Tab/Tab.tsx
index 493758e11fe364..b8b84b45e3c5dc 100644
--- a/packages/mui-joy/src/Tab/Tab.tsx
+++ b/packages/mui-joy/src/Tab/Tab.tsx
@@ -150,7 +150,7 @@ const Tab = React.forwardRef(function Tab(inProps, ref) {
     ...other
   } = props;
 
-  const tabRef = React.useRef<HTMLButtonElement | HTMLAnchorElement | HTMLElement>();
+  const tabRef = React.useRef<HTMLButtonElement | HTMLAnchorElement | HTMLElement | null>(null);
   const handleRef = useForkRef(tabRef, ref) as React.RefCallback<Element>;
 
   const { active, focusVisible, setFocusVisible, selected, getRootProps } = useTab({
diff --git a/packages/mui-joy/src/ToggleButtonGroup/ToggleButtonGroup.tsx b/packages/mui-joy/src/ToggleButtonGroup/ToggleButtonGroup.tsx
index 85282d67ef56aa..82b747a653e911 100644
--- a/packages/mui-joy/src/ToggleButtonGroup/ToggleButtonGroup.tsx
+++ b/packages/mui-joy/src/ToggleButtonGroup/ToggleButtonGroup.tsx
@@ -21,6 +21,7 @@ import useSlot from '../utils/useSlot';
 import { StyledButtonGroup } from '../ButtonGroup/ButtonGroup';
 import ButtonGroupContext from '../ButtonGroup/ButtonGroupContext';
 import ToggleButtonGroupContext from './ToggleButtonGroupContext';
+import { DividerProps } from '../Divider';
 
 interface InternalChangeEventHandler<Value> {
   (event: React.MouseEvent, value: Value | Array<Value> | null): void;
@@ -161,11 +162,11 @@ const ToggleButtonGroup = React.forwardRef(function ToggleButtonGroup<
             }
             const extraProps: Record<string, any> = {};
             if (isMuiElement(child, ['Divider'])) {
-              extraProps.inset = 'inset' in child.props ? child.props.inset : 'context';
+              const childProps = child.props as DividerProps;
+              extraProps.inset = childProps?.inset ?? 'context';
 
               const dividerOrientation = orientation === 'vertical' ? 'horizontal' : 'vertical';
-              extraProps.orientation =
-                'orientation' in child.props ? child.props.orientation : dividerOrientation;
+              extraProps.orientation = childProps?.orientation ?? dividerOrientation;
               extraProps.role = 'presentation';
               extraProps.component = 'span';
             }
diff --git a/packages/mui-joy/src/Tooltip/Tooltip.tsx b/packages/mui-joy/src/Tooltip/Tooltip.tsx
index 7f48d74360228b..e34cf2596709ec 100644
--- a/packages/mui-joy/src/Tooltip/Tooltip.tsx
+++ b/packages/mui-joy/src/Tooltip/Tooltip.tsx
@@ -262,7 +262,7 @@ const Tooltip = React.forwardRef(function Tooltip(inProps, ref) {
 
   const id = useId(idProp);
 
-  const prevUserSelect: React.MutableRefObject<string | undefined> = React.useRef();
+  const prevUserSelect = React.useRef<string | undefined>(undefined);
   const stopTouchInteraction = useEventCallback(() => {
     if (prevUserSelect.current !== undefined) {
       (document.body.style as unknown as { WebkitUserSelect?: string }).WebkitUserSelect =
diff --git a/packages/mui-lab/src/Timeline/Timeline.tsx b/packages/mui-lab/src/Timeline/Timeline.tsx
index f388d198b084d3..f5c7689f857c0d 100644
--- a/packages/mui-lab/src/Timeline/Timeline.tsx
+++ b/packages/mui-lab/src/Timeline/Timeline.tsx
@@ -21,8 +21,8 @@ const useUtilityClasses = (ownerState: OwnerState) => {
   return composeClasses(slots, getTimelineUtilityClass, classes);
 };
 
-const TimelineRoot = styled('ul' as const, {
-  name: 'MuiTimeline' as const,
+const TimelineRoot = styled('ul', {
+  name: 'MuiTimeline',
   slot: 'Root',
   overridesResolver: (props, styles) => {
     const { ownerState } = props;
@@ -62,7 +62,6 @@ const Timeline = React.forwardRef<HTMLUListElement, TimelineProps>(function Time
       <TimelineRoot
         className={clsx(classes.root, className)}
         ownerState={ownerState}
-        // @ts-expect-error TypeScript bug, need to keep unknown for DX
         ref={ref}
         {...other}
       />
diff --git a/packages/mui-lab/src/Timeline/Timeline.types.ts b/packages/mui-lab/src/Timeline/Timeline.types.ts
index 01e9cbf6df0176..22207ea2316e96 100644
--- a/packages/mui-lab/src/Timeline/Timeline.types.ts
+++ b/packages/mui-lab/src/Timeline/Timeline.types.ts
@@ -4,7 +4,7 @@ import { Theme } from '@mui/material/styles';
 import { SxProps } from '@mui/system';
 import { TimelineClasses } from './timelineClasses';
 
-export interface TimelineProps extends StandardProps<React.HTMLAttributes<HTMLUListElement>> {
+export interface TimelineProps extends StandardProps<React.ComponentProps<'ul'>> {
   /**
    * The position where the TimelineContent should appear relative to the time axis.
    * @default 'right'
diff --git a/packages/mui-lab/test/describeConformance.ts b/packages/mui-lab/test/describeConformance.ts
index 4715d91bad6817..5de62933d19dc5 100644
--- a/packages/mui-lab/test/describeConformance.ts
+++ b/packages/mui-lab/test/describeConformance.ts
@@ -5,7 +5,7 @@ import {
 import { ThemeProvider, createTheme } from '@mui/material/styles';
 
 export default function describeConformance(
-  minimalElement: React.ReactElement<any>,
+  minimalElement: React.ReactElement<unknown>,
   getOptions: () => ConformanceOptions,
 ) {
   function getOptionsWithDefaults() {
diff --git a/packages/mui-material-nextjs/src/v13-pagesRouter/pagesRouterV13Document.tsx b/packages/mui-material-nextjs/src/v13-pagesRouter/pagesRouterV13Document.tsx
index 2bc922d62dd059..924b38638f4bfc 100644
--- a/packages/mui-material-nextjs/src/v13-pagesRouter/pagesRouterV13Document.tsx
+++ b/packages/mui-material-nextjs/src/v13-pagesRouter/pagesRouterV13Document.tsx
@@ -37,7 +37,7 @@ export function createGetInitialProps(plugins: Plugin[]) {
 }
 
 export interface DocumentHeadTagsProps {
-  emotionStyleTags: React.ReactElement<any>[];
+  emotionStyleTags: React.ReactElement<unknown>[];
 }
 
 export function DocumentHeadTags(props: DocumentHeadTagsProps) {
diff --git a/packages/mui-material/src/Popper/Popper.spec.tsx b/packages/mui-material/src/Popper/Popper.spec.tsx
index 0431883265da3f..94a4cad381add3 100644
--- a/packages/mui-material/src/Popper/Popper.spec.tsx
+++ b/packages/mui-material/src/Popper/Popper.spec.tsx
@@ -3,7 +3,7 @@ import { Instance } from '@popperjs/core';
 import Tooltip from '@mui/material/Tooltip';
 
 interface Props {
-  children: React.ReactElement<any>;
+  children: React.ReactElement<unknown>;
   value: number;
 }
 
diff --git a/packages/mui-material/src/Slider/useSlider.ts b/packages/mui-material/src/Slider/useSlider.ts
index 55c59e4d442a13..8437f59b13340e 100644
--- a/packages/mui-material/src/Slider/useSlider.ts
+++ b/packages/mui-material/src/Slider/useSlider.ts
@@ -220,7 +220,7 @@ export function useSlider(parameters: UseSliderParameters): UseSliderReturnValue
     value: valueProp,
   } = parameters;
 
-  const touchId = React.useRef<number>();
+  const touchId = React.useRef<number | undefined>(undefined);
   // We can't use the :active browser pseudo-classes.
   // - The active state isn't triggered when clicking on the rail.
   // - The active state isn't transferred when inversing a range slider.
@@ -275,7 +275,7 @@ export function useSlider(parameters: UseSliderParameters): UseSliderReturnValue
   } = useIsFocusVisible();
   const [focusedThumbIndex, setFocusedThumbIndex] = React.useState(-1);
 
-  const sliderRef = React.useRef<HTMLSpanElement>();
+  const sliderRef = React.useRef<HTMLSpanElement | null>(null);
   const handleFocusRef = useForkRef(focusVisibleRef, sliderRef);
   const handleRef = useForkRef(ref, handleFocusRef);
 
@@ -407,7 +407,7 @@ export function useSlider(parameters: UseSliderParameters): UseSliderReturnValue
       changeValue(event, event.target.valueAsNumber);
     };
 
-  const previousIndex = React.useRef<number>();
+  const previousIndex = React.useRef<number | undefined>(undefined);
   let axis = orientation;
   if (isRtl && orientation === 'horizontal') {
     axis += '-reverse';
diff --git a/packages/mui-material/src/styles/styled.spec.tsx b/packages/mui-material/src/styles/styled.spec.tsx
index 8053428c818c6a..b6646c7845c926 100644
--- a/packages/mui-material/src/styles/styled.spec.tsx
+++ b/packages/mui-material/src/styles/styled.spec.tsx
@@ -125,8 +125,8 @@ function Button({
             return styles;
           },
           startIcon: ({ ownerState: { startIcon, endIcon } }) => [
-            startIcon && { marginRight: 8 },
-            endIcon && { marginLeft: 8 },
+            !!startIcon && { marginRight: 8 },
+            !!endIcon && { marginLeft: 8 },
           ],
         },
       },
diff --git a/packages/mui-material/src/useAutocomplete/useAutocomplete.d.ts b/packages/mui-material/src/useAutocomplete/useAutocomplete.d.ts
index 6c74f9c5f251b6..b1717d2db05788 100644
--- a/packages/mui-material/src/useAutocomplete/useAutocomplete.d.ts
+++ b/packages/mui-material/src/useAutocomplete/useAutocomplete.d.ts
@@ -49,7 +49,7 @@ export interface UseAutocompleteProps<
    * Temporary for Joy UI because the parent listbox is the document object
    * TODO v6: Normalize the logic and remove this param.
    */
-  unstable_isActiveElementInListbox?: (listbox: React.RefObject<HTMLElement>) => boolean;
+  unstable_isActiveElementInListbox?: (listbox: React.RefObject<HTMLElement | null>) => boolean;
   /**
    * If `true`, the portion of the selected suggestion that the user hasn't typed,
    * known as the completion string, appears inline after the input cursor in the textbox.
diff --git a/packages/mui-material/test/describeConformance.ts b/packages/mui-material/test/describeConformance.ts
index 4715d91bad6817..5de62933d19dc5 100644
--- a/packages/mui-material/test/describeConformance.ts
+++ b/packages/mui-material/test/describeConformance.ts
@@ -5,7 +5,7 @@ import {
 import { ThemeProvider, createTheme } from '@mui/material/styles';
 
 export default function describeConformance(
-  minimalElement: React.ReactElement<any>,
+  minimalElement: React.ReactElement<unknown>,
   getOptions: () => ConformanceOptions,
 ) {
   function getOptionsWithDefaults() {
diff --git a/packages/mui-styled-engine-sc/src/GlobalStyles/GlobalStyles.d.ts b/packages/mui-styled-engine-sc/src/GlobalStyles/GlobalStyles.d.ts
index 0c2ef1cd47e33f..d1881586abe383 100644
--- a/packages/mui-styled-engine-sc/src/GlobalStyles/GlobalStyles.d.ts
+++ b/packages/mui-styled-engine-sc/src/GlobalStyles/GlobalStyles.d.ts
@@ -8,4 +8,4 @@ export interface GlobalStylesProps<Theme extends object = {}> {
 
 export default function Global<Theme extends object = {}>(
   props: GlobalStylesProps<Theme>,
-): React.ReactElement<any>;
+): React.JSX.Element;
diff --git a/packages/mui-styled-engine-sc/src/GlobalStyles/GlobalStyles.spec.tsx b/packages/mui-styled-engine-sc/src/GlobalStyles/GlobalStyles.spec.tsx
index f253f453661a40..8365b797407f18 100644
--- a/packages/mui-styled-engine-sc/src/GlobalStyles/GlobalStyles.spec.tsx
+++ b/packages/mui-styled-engine-sc/src/GlobalStyles/GlobalStyles.spec.tsx
@@ -25,7 +25,7 @@ export interface GlobalStylesProps {
   styles: SCGlobalStylesProps<Theme>['styles'];
 }
 
-function GlobalStyles(props: GlobalStylesProps): React.ReactElement<any> {
+function GlobalStyles(props: GlobalStylesProps): React.JSX.Element {
   return <SCGlobalStyles {...props} defaultTheme={defaultTheme} />;
 }
 
diff --git a/packages/mui-styled-engine/src/GlobalStyles/GlobalStyles.d.ts b/packages/mui-styled-engine/src/GlobalStyles/GlobalStyles.d.ts
index e746c3036a3a62..a7a7f66f2c78d5 100644
--- a/packages/mui-styled-engine/src/GlobalStyles/GlobalStyles.d.ts
+++ b/packages/mui-styled-engine/src/GlobalStyles/GlobalStyles.d.ts
@@ -8,4 +8,4 @@ export interface GlobalStylesProps<Theme = {}> {
 
 export default function GlobalStyles<Theme = {}>(
   props: GlobalStylesProps<Theme>,
-): React.ReactElement<any>;
+): React.JSX.Element;
diff --git a/packages/mui-styled-engine/src/GlobalStyles/GlobalStyles.spec.tsx b/packages/mui-styled-engine/src/GlobalStyles/GlobalStyles.spec.tsx
index 13b34d7f29b019..142e29f5d4bb75 100644
--- a/packages/mui-styled-engine/src/GlobalStyles/GlobalStyles.spec.tsx
+++ b/packages/mui-styled-engine/src/GlobalStyles/GlobalStyles.spec.tsx
@@ -25,7 +25,7 @@ export interface GlobalStylesProps {
   styles: EmGlobalStylesProps<Theme>['styles'];
 }
 
-function GlobalStyles(props: GlobalStylesProps): React.ReactElement<any> {
+function GlobalStyles(props: GlobalStylesProps): React.JSX.Element {
   return <EmGlobalStyles {...props} defaultTheme={defaultTheme} />;
 }
 
diff --git a/packages/mui-system/src/Stack/createStack.tsx b/packages/mui-system/src/Stack/createStack.tsx
index 0f1f9659ebb952..3f9eb3877487bb 100644
--- a/packages/mui-system/src/Stack/createStack.tsx
+++ b/packages/mui-system/src/Stack/createStack.tsx
@@ -50,7 +50,7 @@ function useThemePropsDefault<T extends {}>(props: T) {
  * > joinChildren([1,2,3], 0)
  * [1,0,2,0,3]
  */
-function joinChildren(children: React.ReactNode, separator: React.ReactElement<any>) {
+function joinChildren(children: React.ReactNode, separator: React.ReactElement<unknown>) {
   const childrenArray = React.Children.toArray(children).filter(Boolean);
 
   return childrenArray.reduce<React.ReactNode[]>((output, child, index) => {
@@ -209,7 +209,7 @@ export default function createStack(
         className={clsx(classes.root, className)}
         {...other}
       >
-        {divider ? joinChildren(children, divider as React.ReactElement<any>) : children}
+        {divider ? joinChildren(children, divider as React.ReactElement<unknown>) : children}
       </StackRoot>
     );
   }) as OverridableComponent<StackTypeMap>;
diff --git a/packages/mui-system/src/Unstable_Grid/createGrid.tsx b/packages/mui-system/src/Unstable_Grid/createGrid.tsx
index e0984a90dd7f23..24c6756da8527b 100644
--- a/packages/mui-system/src/Unstable_Grid/createGrid.tsx
+++ b/packages/mui-system/src/Unstable_Grid/createGrid.tsx
@@ -164,9 +164,12 @@ export default function createGrid(
         {...other}
       >
         {React.Children.map(children, (child) => {
-          if (React.isValidElement(child) && isMuiElement(child, ['Grid'])) {
+          if (
+            React.isValidElement<{ container?: unknown }>(child) &&
+            isMuiElement(child, ['Grid'])
+          ) {
             return React.cloneElement(child, {
-              unstable_level: (child.props as any).unstable_level ?? level + 1,
+              unstable_level: (child.props as GridProps)?.unstable_level ?? level + 1,
             } as GridProps);
           }
           return child;
diff --git a/packages/mui-system/src/cssVars/createCssVarsProvider.d.ts b/packages/mui-system/src/cssVars/createCssVarsProvider.d.ts
index 00737cb84b8ee5..dd508d52640bc6 100644
--- a/packages/mui-system/src/cssVars/createCssVarsProvider.d.ts
+++ b/packages/mui-system/src/cssVars/createCssVarsProvider.d.ts
@@ -90,7 +90,7 @@ export interface CreateCssVarsProviderResult<
         disableStyleSheetGeneration?: boolean;
       }
     >,
-  ) => React.ReactElement<any>;
+  ) => React.JSX.Element;
   useColorScheme: () => ColorSchemeContextValue<ColorScheme>;
   getInitColorSchemeScript: typeof InitColorSchemeScript;
 }
diff --git a/packages/mui-system/test/describeConformance.ts b/packages/mui-system/test/describeConformance.ts
index aa876199917f98..125489ae430922 100644
--- a/packages/mui-system/test/describeConformance.ts
+++ b/packages/mui-system/test/describeConformance.ts
@@ -5,7 +5,7 @@ import {
 import { ThemeProvider, createTheme } from '@mui/system';
 
 export default function describeConformance(
-  minimalElement: React.ReactElement<any>,
+  minimalElement: React.ReactElement<unknown>,
   getOptions: () => ConformanceOptions,
 ) {
   function getOptionsWithDefaults() {
diff --git a/packages/mui-utils/src/getValidReactChildren/getValidReactChildren.ts b/packages/mui-utils/src/getValidReactChildren/getValidReactChildren.ts
index ea1132e5fc32ba..f7e33f77d73551 100644
--- a/packages/mui-utils/src/getValidReactChildren/getValidReactChildren.ts
+++ b/packages/mui-utils/src/getValidReactChildren/getValidReactChildren.ts
@@ -9,5 +9,5 @@ import * as React from 'react';
 export default function getValidReactChildren(children: React.ReactNode) {
   return React.Children.toArray(children).filter((child) =>
     React.isValidElement(child),
-  ) as React.ReactElement<any>[];
+  ) as React.ReactElement<unknown>[];
 }
diff --git a/packages/mui-utils/src/setRef/setRef.spec.tsx b/packages/mui-utils/src/setRef/setRef.spec.tsx
index c9c90757c0d8af..a5be046dd0914a 100644
--- a/packages/mui-utils/src/setRef/setRef.spec.tsx
+++ b/packages/mui-utils/src/setRef/setRef.spec.tsx
@@ -2,7 +2,7 @@ import * as React from 'react';
 import setRef from './setRef';
 
 function MyRef() {
-  const ref = React.useRef<HTMLDivElement>();
+  const ref = React.useRef<HTMLDivElement | null>(null);
 
   const handleRef = React.useCallback((node: HTMLDivElement) => {
     setRef(ref, node);
diff --git a/packages/test-utils/src/createRenderer.tsx b/packages/test-utils/src/createRenderer.tsx
index 8c3785247c75af..627783c6693c7a 100644
--- a/packages/test-utils/src/createRenderer.tsx
+++ b/packages/test-utils/src/createRenderer.tsx
@@ -281,8 +281,12 @@ export interface MuiRenderToStringResult {
   hydrate(): MuiRenderResult;
 }
 
+interface DataAttributes {
+  [key: `data-${string}`]: string;
+}
+
 function render(
-  element: React.ReactElement<any>,
+  element: React.ReactElement<DataAttributes>,
   configuration: ClientRenderConfiguration,
 ): MuiRenderResult {
   const { container, hydrate, wrapper } = configuration;
@@ -317,7 +321,7 @@ function render(
 }
 
 function renderToString(
-  element: React.ReactElement<any>,
+  element: React.ReactElement<DataAttributes>,
   configuration: ServerRenderConfiguration,
 ): { container: HTMLElement; hydrate(): MuiRenderResult } {
   const { container, wrapper: Wrapper } = configuration;
@@ -439,9 +443,9 @@ function createClock(defaultMode: 'fake' | 'real', config: ClockConfig): Clock {
 
 interface Renderer {
   clock: Clock;
-  render(element: React.ReactElement<any>, options?: RenderOptions): MuiRenderResult;
+  render(element: React.ReactElement<DataAttributes>, options?: RenderOptions): MuiRenderResult;
   renderToString(
-    element: React.ReactElement<any>,
+    element: React.ReactElement<DataAttributes>,
     options?: RenderOptions,
   ): MuiRenderToStringResult;
 }
@@ -581,7 +585,7 @@ export function createRenderer(globalOptions: CreateRendererOptions = {}): Rende
 
   return {
     clock,
-    render(element: React.ReactElement<any>, options: RenderOptions = {}) {
+    render(element: React.ReactElement<DataAttributes>, options: RenderOptions = {}) {
       if (!prepared) {
         throw new Error(
           'Unable to finish setup before `render()` was called. ' +
@@ -596,7 +600,7 @@ export function createRenderer(globalOptions: CreateRendererOptions = {}): Rende
         wrapper: createWrapper(options),
       });
     },
-    renderToString(element: React.ReactElement<any>, options: RenderOptions = {}) {
+    renderToString(element: React.ReactElement<DataAttributes>, options: RenderOptions = {}) {
       if (!prepared) {
         throw new Error(
           'Unable to finish setup before `render()` was called. ' +