diff --git a/packages/injected/src/selectorGenerator.ts b/packages/injected/src/selectorGenerator.ts
index 8410d7dc7d..a7dad6644f 100644
--- a/packages/injected/src/selectorGenerator.ts
+++ b/packages/injected/src/selectorGenerator.ts
@@ -578,6 +578,25 @@ function escapeNodeName(node: Node): string {
 }
 
 function escapeClassName(className: string): string {
-  // We are escaping it for document.querySelectorAll, not for usage in CSS file.
-  return className.replace(/[:\.]/g, char => '\\' + char);
+  // We are escaping class names for document.querySelectorAll by following CSS.escape() rules.
+  let result = '';
+  for (let i = 0; i < className.length; i++)
+    result += cssEscapeCharacter(className, i);
+  return result;
+}
+
+function cssEscapeCharacter(s: string, i: number): string {
+  // https://drafts.csswg.org/cssom/#serialize-an-identifier
+  const c = s.charCodeAt(i);
+  if (c === 0x0000)
+    return '\uFFFD';
+  if ((c >= 0x0001 && c <= 0x001f) ||
+      (c >= 0x0030 && c <= 0x0039 && (i === 0 || (i === 1 && s.charCodeAt(0) === 0x002d))))
+    return '\\' + c.toString(16) + ' ';
+  if (i === 0 && c === 0x002d && s.length === 1)
+    return '\\' + s.charAt(i);
+  if (c >= 0x0080 || c === 0x002d || c === 0x005f || (c >= 0x0030 && c <= 0x0039) ||
+      (c >= 0x0041 && c <= 0x005a) || (c >= 0x0061 && c <= 0x007a))
+    return s.charAt(i);
+  return '\\' + s.charAt(i);
 }
diff --git a/tests/library/selector-generator.spec.ts b/tests/library/selector-generator.spec.ts
index 68f0d2fccf..a78fb15a5c 100644
--- a/tests/library/selector-generator.spec.ts
+++ b/tests/library/selector-generator.spec.ts
@@ -311,13 +311,14 @@ it.describe('selector generator', () => {
           <c>
           </c>
         </b>
-        <b class="foo bar.baz">
+        <b>
           <c mark=1></c>
         </b>
       </div>
       <div><b class="foo"></b></div>
     `);
-    expect(await generate(page, 'c[mark="1"]')).toBe('.foo.bar\\.baz > c');
+    await page.$eval('[mark="1"]', c => c.parentElement.className = 'foo 12.bar.baz[&x]-_?"\'');
+    expect(await generate(page, 'c[mark="1"]')).toBe(`.foo.\\31 2\\.bar\\.baz\\[\\&x\\]-_\\?\\"\\' > c`);
   });
 
   it('should properly join child selectors under nested ordinals', async ({ page }) => {