trying helpful idea :)

assets/javascripts/image-tag-whitelist.js

@@ -1,3 +1,12 @@
-(function () {
-    Discourse.Markdown.whiteListTag('color', 'center', 'anchor', 'font', 'table', 'tr', 'td', 'span');
-}).call(this);
+import WhiteLister from 'pretty-text/white-lister';
+
+export default {
+  name: 'dropcap',
+
+  initialize() {
+    const whiteLister = new WhiteLister();
+    whiteLister.whiteListFeature("dropcap", [
+      'span.dropcap'
+    ]);
+  }
+};