diff --git a/src/scriptlets/inject-css-in-shadow-dom.js b/src/scriptlets/inject-css-in-shadow-dom.js
index 656deee5a..c9f71271b 100644
--- a/src/scriptlets/inject-css-in-shadow-dom.js
+++ b/src/scriptlets/inject-css-in-shadow-dom.js
@@ -1,5 +1,6 @@
 import {
     hit,
+    logMessage,
     hijackAttachShadow,
 } from '../helpers/index';
 
@@ -11,10 +12,10 @@ import {
  *
  * **Syntax**
  * ```
- * example.org#%#//scriptlet('inject-css-in-shadow-dom', cssRule)
+ * example.org#%#//scriptlet('inject-css-in-shadow-dom', cssText)
  * ```
  *
- * - `cssRule` - required, string of comma-separated css rules
+ * - `cssText` - required, string of comma-separated css rules
  *
  * **Examples**
  * ```
@@ -22,28 +23,43 @@ import {
  * example.org#%#//scriptlet('inject-css-in-shadow-dom', '#advertisement { display: none !important; }')
  *
  * ! apply multiple css rules
- * example.org#%#//scriptlet('inject-css-in-shadow-dom', '#advertisement { display: none !important; }, #content { margin-top: 0 !important; }')
+ * example.org#%#//scriptlet('inject-css-in-shadow-dom', '#advertisement { display: none !important; }|#content { margin-top: 0 !important; }')
  * ```
  */
 /* eslint-enable max-len */
 
-export function injectCssInShadowDom(source, cssRule) {
+export function injectCssInShadowDom(source, cssText) {
     // do nothing if browser does not support ShadowRoot
     // https://developer.mozilla.org/en-US/docs/Web/API/ShadowRoot
     if (!Element.prototype.attachShadow) {
         return;
     }
 
-    const parsedStyleRules = cssRule.split(',');
+    const parsedStyleRules = cssText.split('|');
     const callback = (shadowRoot) => {
         const stylesheet = new CSSStyleSheet();
 
         // fill stylesheet with rules
-        parsedStyleRules.forEach((rule) => stylesheet.insertRule(rule));
+        parsedStyleRules.forEach((rule) => {
+            try {
+                stylesheet.insertRule(rule);
+            } catch {
+                logMessage(source, `Failed to parse the rule: ${rule}`);
+            }
+        });
 
         // attach stylesheet to shadow root so the whole subtree would be affected
-        // https://developer.mozilla.org/en-US/docs/Web/API/Document/adoptedStyleSheets
-        shadowRoot.adoptedStyleSheets = [...shadowRoot.adoptedStyleSheets, stylesheet];
+        if (shadowRoot.adoptedStyleSheets) {
+            // adoptedStyleSheets is not yet supported by Safari
+            // https://developer.mozilla.org/en-US/docs/Web/API/Document/adoptedStyleSheets
+            shadowRoot.adoptedStyleSheets = [...shadowRoot.adoptedStyleSheets, stylesheet];
+        } else {
+            const styleTag = document.createElement('style');
+            styleTag.innerText = cssText;
+            shadowRoot.appendChild(styleTag);
+        }
+
+        hit(source);
     };
 
     hijackAttachShadow(window, callback);
@@ -55,5 +71,6 @@ injectCssInShadowDom.names = [
 
 injectCssInShadowDom.injections = [
     hit,
+    logMessage,
     hijackAttachShadow,
 ];
diff --git a/tests/scriptlets/index.test.js b/tests/scriptlets/index.test.js
index aa6410e30..e27d0be95 100644
--- a/tests/scriptlets/index.test.js
+++ b/tests/scriptlets/index.test.js
@@ -50,3 +50,4 @@ import './trusted-set-cookie.test';
 import './trusted-replace-fetch-response.test';
 import './trusted-set-local-storage-item.test';
 import './trusted-set-constant.test';
+import './inject-css-in-shadow-dom.test';
diff --git a/tests/scriptlets/inject-css-in-shadow-dom.test.js b/tests/scriptlets/inject-css-in-shadow-dom.test.js
new file mode 100644
index 000000000..31119aa39
--- /dev/null
+++ b/tests/scriptlets/inject-css-in-shadow-dom.test.js
@@ -0,0 +1,63 @@
+/* eslint-disable no-underscore-dangle */
+import { runScriptlet, clearGlobalProps } from '../helpers';
+
+const { test, module } = QUnit;
+const name = 'inject-css-in-shadow-dom';
+
+const HOST_ID = 'host';
+const TARGET_ID = 'target';
+const TARGET_CSS_PROP = 'color';
+const TARGET_CSS_VALUE = 'rgb(255, 0, 0)';
+const CSS_TEXT = `#target { ${TARGET_CSS_PROP}: ${TARGET_CSS_VALUE}  !important}`;
+// const CSS_TEXT = '#target { color: red  !important}';
+
+const appendTarget = (parent) => {
+    const target = document.createElement('h1');
+    target.id = TARGET_ID;
+    target.innerText = 'Target element';
+    return parent.appendChild(target);
+};
+
+const appendHost = () => {
+    const host = document.createElement('div');
+    host.id = HOST_ID;
+    return document.body.appendChild(host);
+};
+
+const removeHost = () => document.getElementById(HOST_ID)?.remove();
+
+const beforeEach = () => {
+    window.__debug = () => {
+        window.hit = 'FIRED';
+    };
+};
+
+const afterEach = () => {
+    clearGlobalProps('hit', '__debug');
+    removeHost();
+};
+
+module(name, { beforeEach, afterEach });
+
+// some browsers do not support ShadowRoot
+// for example, Firefox 52 which is used for browserstack tests
+// https://developer.mozilla.org/en-US/docs/Web/API/ShadowRoot
+const isSupported = typeof Element.prototype.attachShadow !== 'undefined';
+
+if (!isSupported) {
+    test('unsupported', (assert) => {
+        assert.ok(true, 'Browser does not support it');
+    });
+} else {
+    test('styles applied to shadow root subtree with adoptedStyleSheets', (assert) => {
+        runScriptlet(name, [CSS_TEXT]);
+
+        const host = appendHost();
+        const shadowRoot = host.attachShadow({ mode: 'closed' });
+        appendTarget(shadowRoot);
+
+        const target = shadowRoot.getElementById(TARGET_ID);
+        assert.strictEqual(getComputedStyle(target)[TARGET_CSS_PROP], TARGET_CSS_VALUE, 'style was applied on target');
+        assert.strictEqual(window.hit, 'FIRED', 'hit function was executed');
+    });
+}