Wim Leers
--- a/core/config/schema/core.data_types.schema.yml
+++ b/core/config/schema/core.data_types.schema.yml
+          # @todo use \Symfony\Component\Validator\Constraints\ImageValidator?
        url:
-          type: string
+          # TRICKY: this is only part of config due to theme_get_setting() storing the URL computed from `path` in
+          # config; it's not actually configurable. Hence there's no need for actual validation: if `path` is set, then
+          # this will be (re)computed in theme_get_setting().
+          type: uri
          label: 'URL'
+          requiredKey: false
+          nullable: true
        use_default:
          type: boolean
          label: 'Use the default shortcut icon supplied by the theme'
+            # If this is `true`, then `mimetype` and `path` are indeed optional keys. But if this is `false`, then they
+            # MUST be specified.
+            # @todo Figure out how to conditionally require other keys … without changing the config schema structure
--- a/core/config/schema/core.data_types.schema.yml
+++ b/core/config/schema/core.data_types.schema.yml
+  label: 'Shortcut icon settings'
+  mapping:
+    use_default:
+      type: boolean
+      label: 'Use the default shortcut icon supplied by the theme'
+      constraints: { }
+
+theme_settings_favicon.0:
+  type: 'theme_settings_favicon.*'
+  label: 'Shortcut icon settings'
+  mapping:
+    mimetype:
+      type: string
+      label: 'MIME type'
+      requiredKey: false
+      nullable: true
--- a/core/config/schema/core.data_types.schema.yml
+++ b/core/config/schema/core.data_types.schema.yml
+      type: boolean
+      label: 'Use the default shortcut icon supplied by the theme'
+
+theme_settings_favicon.0:
+  type: 'theme_settings_favicon.*'
+  label: 'Shortcut icon settings'
+  mapping:
+    mimetype:
+      type: string
+      label: 'MIME type'
+      nullable: true
+      constraints:
+        Regex:
+          pattern: '/^image\/[a-z_\-\+]+$/'
+          message: "%value is not a valid image MIME type."
+    path: