diff --git a/.changeset/nine-fishes-itch.md b/.changeset/nine-fishes-itch.md
new file mode 100644
index 00000000000..24dc387eb65
--- /dev/null
+++ b/.changeset/nine-fishes-itch.md
@@ -0,0 +1,5 @@
+---
+"@itwin/itwinui-react": patch
+---
+
+Fixed an issue in older Safari versions where visually-hidden styles inside `ProgressRadial` were not being applied.
diff --git a/packages/itwinui-react/src/core/utils/components/ShadowRoot.tsx b/packages/itwinui-react/src/core/utils/components/ShadowRoot.tsx
index 8d6e07ebb88..40a85d989f8 100644
--- a/packages/itwinui-react/src/core/utils/components/ShadowRoot.tsx
+++ b/packages/itwinui-react/src/core/utils/components/ShadowRoot.tsx
@@ -4,6 +4,7 @@
  *--------------------------------------------------------------------------------------------*/
 import * as React from 'react';
 import * as ReactDOM from 'react-dom';
+import { useLatestRef, useLayoutEffect } from '../hooks/index.js';
 
 const isBrowser = typeof document !== 'undefined';
 const supportsDSD =
@@ -11,47 +12,16 @@ const supportsDSD =
 const supportsAdoptedStylesheets =
   isBrowser && 'adoptedStyleSheets' in Document.prototype;
 
+type ShadowRootProps = { children: React.ReactNode; css?: string };
+
 /**
  * Wrapper around `<template>` element that attaches shadow root to its parent
  * and moves its children into the shadow root.
  *
  * @private
  */
-export const ShadowRoot = ({
-  children,
-  css,
-}: {
-  children: React.ReactNode;
-  css?: string;
-}) => {
-  const [shadowRoot, setShadowRoot] = React.useState<ShadowRoot>();
+export const ShadowRoot = ({ children, css }: ShadowRootProps) => {
   const isFirstRender = useIsFirstRender();
-  const styleSheet = React.useRef<CSSStyleSheet>();
-
-  const attachShadowRef = React.useCallback(
-    (template: HTMLTemplateElement | null) => {
-      const parent = template?.parentElement;
-      if (!template || !parent) {
-        return;
-      }
-      if (parent.shadowRoot) {
-        parent.shadowRoot.replaceChildren(); // Remove previous shadowroot content
-      }
-      queueMicrotask(() => {
-        const shadow =
-          parent.shadowRoot || parent.attachShadow({ mode: 'open' });
-
-        if (css && supportsAdoptedStylesheets) {
-          styleSheet.current ||= new CSSStyleSheet();
-          styleSheet.current.replaceSync(css);
-          shadow.adoptedStyleSheets = [styleSheet.current];
-        }
-
-        ReactDOM.flushSync(() => setShadowRoot(shadow));
-      });
-    },
-    [css],
-  );
 
   if (!isBrowser) {
     return (
@@ -68,19 +38,91 @@ export const ShadowRoot = ({
     return null;
   }
 
-  return (
-    <>
-      {shadowRoot ? (
-        ReactDOM.createPortal(children, shadowRoot)
-      ) : (
-        <template ref={attachShadowRef} />
-      )}
-    </>
+  return <ClientShadowRoot css={css}>{children}</ClientShadowRoot>;
+};
+
+// ----------------------------------------------------------------------------
+
+const ClientShadowRoot = ({ children, css }: ShadowRootProps) => {
+  const templateRef = React.useRef<HTMLTemplateElement>(null);
+  const shadowRoot = useShadowRoot(templateRef, { css });
+
+  // fallback to <style> tag if adoptedStyleSheets is not supported
+  const fallbackCss =
+    !supportsAdoptedStylesheets && css ? <style>{css}</style> : null;
+
+  return shadowRoot ? (
+    ReactDOM.createPortal(
+      <>
+        {fallbackCss}
+        {children}
+      </>,
+      shadowRoot,
+    )
+  ) : (
+    <template ref={templateRef} />
   );
 };
 
 // ----------------------------------------------------------------------------
 
+/**
+ * Given a ref, this hook will return a shadowRoot attached to its parent element.
+ *
+ * The css will be added to the shadowRoot using `adoptedStyleSheets` (if supported).
+ */
+function useShadowRoot(
+  templateRef: React.RefObject<HTMLElement>,
+  { css = '' },
+) {
+  const [shadowRoot, setShadowRoot] = React.useState<ShadowRoot | null>(null);
+  const styleSheet = React.useRef<CSSStyleSheet>();
+  const latestCss = useLatestRef(css);
+
+  useLayoutEffect(() => {
+    const parent = templateRef.current?.parentElement;
+    if (!parent) {
+      return;
+    }
+
+    if (parent.shadowRoot) {
+      parent.shadowRoot.replaceChildren(); // Remove previous shadowroot content
+    }
+
+    const shadow = parent.shadowRoot || parent.attachShadow({ mode: 'open' });
+
+    if (supportsAdoptedStylesheets) {
+      // create an empty stylesheet and add it to the shadowRoot
+      const currentWindow = shadow.ownerDocument.defaultView || globalThis;
+      styleSheet.current = new currentWindow.CSSStyleSheet();
+      shadow.adoptedStyleSheets = [styleSheet.current];
+
+      // add the CSS immediately to avoid FOUC (one-time)
+      if (latestCss.current) {
+        styleSheet.current.replaceSync(latestCss.current);
+      }
+    }
+
+    queueMicrotask(() => {
+      // Flush the state immediately to ensure layout measurements in parent component are correct
+      ReactDOM.flushSync(() => setShadowRoot(shadow));
+    });
+
+    return () => void setShadowRoot(null);
+  }, [templateRef, latestCss]);
+
+  // Synchronize `css` with contents of the existing stylesheet
+  useLayoutEffect(() => {
+    if (css && supportsAdoptedStylesheets) {
+      styleSheet.current?.replaceSync(css);
+    }
+  }, [css]);
+
+  return shadowRoot;
+}
+
+// ----------------------------------------------------------------------------
+
 function useIsFirstRender() {
   const [isFirstRender, setIsFirstRender] = React.useState(true);
   React.useEffect(() => setIsFirstRender(false), []);