3 files changed, 15 insertions, 2 deletions
diff --git a/Userland/Libraries/LibWeb/CSS/StyleSheetList.cpp b/Userland/Libraries/LibWeb/CSS/StyleSheetList.cpp
index 42e365df20..8337be16c3 100644
--- a/Userland/Libraries/LibWeb/CSS/StyleSheetList.cpp
+++ b/Userland/Libraries/LibWeb/CSS/StyleSheetList.cpp
@@ -18,4 +18,15 @@ StyleSheetList::StyleSheetList(DOM::Document& document)
 {
 }
 
+// https://drafts.csswg.org/cssom/#ref-for-dfn-supported-property-indices%E2%91%A1
+bool StyleSheetList::is_supported_property_index(u32 index) const
+{
+    // The object’s supported property indices are the numbers in the range zero to one less than the number of CSS style sheets represented by the collection.
+    // If there are no such CSS style sheets, then there are no supported property indices.
+    if (m_sheets.is_empty())
+        return false;
+
+    return index < m_sheets.size();
+}
+
 }
diff --git a/Userland/Libraries/LibWeb/CSS/StyleSheetList.h b/Userland/Libraries/LibWeb/CSS/StyleSheetList.h
index 5b52008f3a..04ccf51ebb 100644
--- a/Userland/Libraries/LibWeb/CSS/StyleSheetList.h
+++ b/Userland/Libraries/LibWeb/CSS/StyleSheetList.h
@@ -37,6 +37,8 @@ public:
 
     size_t length() const { return m_sheets.size(); }
 
+    bool is_supported_property_index(u32) const;
+
 private:
     explicit StyleSheetList(DOM::Document&);
 
diff --git a/Userland/Libraries/LibWeb/CSS/StyleSheetList.idl b/Userland/Libraries/LibWeb/CSS/StyleSheetList.idl
index 27460affa4..15dc0d41f6 100644
--- a/Userland/Libraries/LibWeb/CSS/StyleSheetList.idl
+++ b/Userland/Libraries/LibWeb/CSS/StyleSheetList.idl
@@ -1,5 +1,5 @@
+[Exposed=Window]
 interface StyleSheetList {
-    // FIXME: item() should be a WebIDL "getter"
-    CSSStyleSheet? item(unsigned long index);
+    getter CSSStyleSheet? item(unsigned long index);
     readonly attribute unsigned long length;
 };