fixed it

assets/javascripts/lib/discourse-markdown/span-tag-whitelist.js.es6

@@ -0,0 +1,15 @@
+// start point  
+export default {
+  name: 'dropcap',
+  initialize() {
+    setup(helper);
+  }
+};
+
+export function setup(helper) {
+  helper.registerOptions((opts, siteSettings)=>{
+    opts.features['dropcaps'] = true; //!!siteSettings.dropcaps_enabled;
+  });
+
+  helper.whiteList([ 'span.dropcaps', 'span.sclass' ]);
+}