diff --git a/crates/obscura-js/js/bootstrap.js b/crates/obscura-js/js/bootstrap.js
index 46637cc..f6a8d4a 100644
--- a/crates/obscura-js/js/bootstrap.js
+++ b/crates/obscura-js/js/bootstrap.js
@@ -6049,6 +6049,12 @@ globalThis.DocumentType = DocumentType;
 globalThis.Node = Node;
 globalThis.Element = Element;
 globalThis.Document = Document;
+// CSSStyleDeclaration is the type of element.style and getComputedStyle(); it is
+// pre-declared non-enumerable in _preHideInternals, but unlike the other WebIDL
+// interfaces it had no value assignment, leaving `window.CSSStyleDeclaration`
+// undefined (so `el.style instanceof CSSStyleDeclaration` threw). Assigning here
+// only fills the value; the property stays enumerable:false, matching Chrome.
+globalThis.CSSStyleDeclaration = CSSStyleDeclaration;
 globalThis.XPathResult = globalThis.XPathResult || class XPathResult {};
 Object.assign(globalThis.XPathResult, {
   ANY_TYPE: 0,
diff --git a/crates/obscura-js/src/runtime.rs b/crates/obscura-js/src/runtime.rs
index 5d1a14a..69dec4a 100644
--- a/crates/obscura-js/src/runtime.rs
+++ b/crates/obscura-js/src/runtime.rs
@@ -3552,6 +3552,20 @@ mod tests {
         assert_eq!(kb, serde_json::json!(true));
     }
 
+    #[test]
+    fn cssstyledeclaration_is_a_usable_global_interface() {
+        // CSSStyleDeclaration was pre-declared non-enumerable but never assigned
+        // a value (the only WebIDL interface missing its globalThis.X = X line),
+        // so it was `undefined` while `'CSSStyleDeclaration' in window` was true,
+        // and `el.style instanceof CSSStyleDeclaration` threw. It must be a real
+        // constructor, non-enumerable like a browser, and the type of .style.
+        let mut rt = setup_runtime("<html><body></body></html>");
+        let v = rt
+            .evaluate("(function(){var d=Object.getOwnPropertyDescriptor(window,'CSSStyleDeclaration');return (typeof window.CSSStyleDeclaration)+'|'+(document.body.style instanceof CSSStyleDeclaration)+'|'+(d?d.enumerable:'missing');})()")
+            .unwrap();
+        assert_eq!(v, serde_json::json!("function|true|false"));
+    }
+
     #[test]
     fn test_create_event_unknown_type_returns_event() {
         let mut rt = setup_runtime("<html><body></body></html>");