Add ::details-content (web-platform-dx#2130)

Author: jamesnw

features/details-content.yml

@@ -0,0 +1,6 @@
+name: ::details-content
+description: The `::details-content` pseudo-element selects the expandable content of a `<details>` element, excluding the `<summary>`.
+spec: https://drafts.csswg.org/css-pseudo-4/#details-content-pseudo
+group: selectors
+compat_features:
+  - css.selectors.details-content

features/details-content.yml.dist

@@ -0,0 +1,8 @@
+# Generated from: details-content.yml
+# Do not edit this file by hand. Edit the source file instead!
+
+status:
+  baseline: false
+  support: {}
+compat_features:
+  - css.selectors.details-content