diff --git a/api/HTMLTemplateElement.json b/api/HTMLTemplateElement.json
index 1879b329b971d9..b5d8530f377b16 100644
--- a/api/HTMLTemplateElement.json
+++ b/api/HTMLTemplateElement.json
@@ -303,6 +303,48 @@
             "deprecated": false
           }
         }
+      },
+      "shadowRootSlotAssignment": {
+        "__compat": {
+          "spec_url": "https://html.spec.whatwg.org/multipage/scripting.html#dom-template-shadowrootslotassignment",
+          "tags": [
+            "web-features:declarative-shadow-dom"
+          ],
+          "support": {
+            "chrome": {
+              "version_added": false,
+              "impl_url": "https://crbug.com/493315747"
+            },
+            "chrome_android": "mirror",
+            "edge": "mirror",
+            "firefox": {
+              "version_added": "preview",
+              "flags": [
+                {
+                  "type": "preference",
+                  "name": "dom.shadowdom.shadowRootSlotAssignment.enabled",
+                  "value_to_set": "true"
+                }
+              ]
+            },
+            "firefox_android": "mirror",
+            "oculus": "mirror",
+            "opera": "mirror",
+            "opera_android": "mirror",
+            "safari": {
+              "version_added": "preview"
+            },
+            "safari_ios": "mirror",
+            "samsunginternet_android": "mirror",
+            "webview_android": "mirror",
+            "webview_ios": "mirror"
+          },
+          "status": {
+            "experimental": true,
+            "standard_track": true,
+            "deprecated": false
+          }
+        }
       }
     }
   }
diff --git a/html/elements/template.json b/html/elements/template.json
index 54c5c6aabfbe7a..ca4009debcab58 100644
--- a/html/elements/template.json
+++ b/html/elements/template.json
@@ -278,6 +278,47 @@
               "deprecated": false
             }
           }
+        },
+        "shadowrootslotassignment": {
+          "__compat": {
+            "spec_url": "https://html.spec.whatwg.org/multipage/scripting.html#attr-template-shadowrootslotassignment",
+            "tags": [
+              "web-features:declarative-shadow-dom"
+            ],
+            "support": {
+              "chrome": {
+                "version_added": false
+              },
+              "chrome_android": "mirror",
+              "edge": "mirror",
+              "firefox": {
+                "version_added": "preview",
+                "flags": [
+                  {
+                    "type": "preference",
+                    "name": "dom.shadowdom.shadowRootSlotAssignment.enabled",
+                    "value_to_set": "true"
+                  }
+                ]
+              },
+              "firefox_android": "mirror",
+              "oculus": "mirror",
+              "opera": "mirror",
+              "opera_android": "mirror",
+              "safari": {
+                "version_added": false
+              },
+              "safari_ios": "mirror",
+              "samsunginternet_android": "mirror",
+              "webview_android": "mirror",
+              "webview_ios": "mirror"
+            },
+            "status": {
+              "experimental": true,
+              "standard_track": true,
+              "deprecated": false
+            }
+          }
         }
       }
     }