diff --git a/css/selectors.json b/css/selectors.json
index f991ab365..480cbacb7 100644
--- a/css/selectors.json
+++ b/css/selectors.json
@@ -569,6 +569,15 @@
     "status": "standard",
     "mdn_url": "https://developer.mozilla.org/docs/Web/CSS/:only-of-type"
   },
+  ":open": {
+    "syntax": ":open",
+    "groups": [
+      "Pseudo-classes",
+      "Selectors"
+    ],
+    "status": "standard",
+    "mdn_url": "https://developer.mozilla.org/docs/Web/CSS/:open"
+  },
   ":optional": {
     "syntax": ":optional",
     "groups": [