Add element-scoped view transitions (#3681)

Fixes #3423

Author: ddbeck

features/view-transitions-element-scoped.yml

@@ -0,0 +1,13 @@
+name: Element-scoped view transitions
+description: >
+  The `startViewTransition()` method of an `Element` object starts a view transition that affects only that element's DOM tree.
+  The `contain: view-transition` CSS declaration contains the layout of the tree to be transitioned.
+  You can use this to run separate elements' transitions concurrently.
+spec: https://github.com/WICG/view-transitions/blob/main/scoped-transitions.md
+group: view-transitions
+# Expecting:
+# compat_features:
+#  - api.Element.startViewTransition
+#  - css.properties.contain.view-transition
+# This feature is subordinate to view-transitions. It should probably merge into
+# it one day.

features/view-transitions-element-scoped.yml.dist

@@ -0,0 +1,6 @@
+# Generated from: view-transitions-element-scoped.yml
+# Do not edit this file by hand. Edit the source file instead!
+
+status:
+  baseline: false
+  support: {}

scripts/specs.ts

@@ -175,6 +175,10 @@ const defaultAllowlist: allowlistItem[] = [
     [
         "https://www.w3.org/TR/2019/WD-feature-policy-1-20190416/",
         "Allowed because feature policy was replaced by permissions policy."
+    ],
+    [
+        "https://github.com/WICG/view-transitions/blob/main/scoped-transitions.md",
+        "Allowed until there's proper spec text. Follow https://github.com/w3c/csswg-drafts/issues/9890 for details."
     ]
 ];