diff --git a/dom/base/nsContentUtils.cpp b/dom/base/nsContentUtils.cpp
index 34c7d23b86..8612e76df4 100644
--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -9787,3 +9787,19 @@ nsContentUtils::AttemptLargeAllocationLoad(nsIHttpChannel* aChannel)
 
   return reloadSucceeded;
 }
+
+/* static */ bool
+nsContentUtils::IsLocalRefURL(const nsString& aString)
+{
+  // Find the first non-"C0 controls + space" character.
+  const char16_t* current = aString.get();
+  for (; *current != '\0'; current++) {
+    if (*current > 0x20) {
+      // if the first non-"C0 controls + space" character is '#', this is a
+      // local-ref URL.
+      return *current == '#';
+    }
+  }
+
+  return false;
+}
\ No newline at end of file
diff --git a/dom/base/nsContentUtils.h b/dom/base/nsContentUtils.h
index 9ae6d2155d..c255f813af 100644
--- a/dom/base/nsContentUtils.h
+++ b/dom/base/nsContentUtils.h
@@ -2730,6 +2730,13 @@ public:
 
   static bool AttemptLargeAllocationLoad(nsIHttpChannel* aChannel);
 
+  /**
+   * Detect whether a string is a (CSS) local-url.
+   * https://drafts.csswg.org/css-values/#local-urls
+   */
+  static bool
+  IsLocalRefURL(const nsString& aString);
+
 private:
   static bool InitializeEventTable();