feat(css): add schema validation and supplementary documentation for custom properties

css/properties.md

@@ -117,3 +117,7 @@ There are 3 more properties that are optional:
 * `mdn_url` (string): a URL linking to the property's page on MDN. This URL must omit the localization part of the URL (such as `en-US/`).
 * `stacking` (boolean): Whether or not the property creates a stacking context. See [stacking context](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_positioned_layout/Understanding_z-index/Stacking_context) for details.
 * `alsoAppliesTo` (enum): To which elements the property also applies to. See the schema for [a list of enums](https://github.com/mdn/data/blob/main/css/properties.schema.json#L253).
+
+### Custom Properties
+
+Custom properties are a special case, since they have variable keys. These keys must be prefixed by two dashes, (e.g. `--brand-color`), and are validated by a regular expression in the [`patternProperties` section of the schema](https://github.com/mdn/data/blob/main/css/properties.schema.json#L472).

css/properties.schema.json

@@ -468,5 +468,10 @@
         "$ref": "#/definitions/mdn_url"
       }
     }
+  },
+  "patternProperties": {
+    "^--": {
+      "$ref": "#/definitions/additionalProperties"
+    }
   }
 }