diff --git a/pxtsim/runtime.ts b/pxtsim/runtime.ts
index 3a95257e847..3b274aa58bd 100644
--- a/pxtsim/runtime.ts
+++ b/pxtsim/runtime.ts
@@ -148,7 +148,7 @@ namespace pxsim {
             return Promise.all(values.map(v => mapper(v)));
         }
 
-        export  function promiseMapAllSeries<T, V>(values: T[], mapper: (obj: T) => Promise<V>): Promise<V[]> {
+        export function promiseMapAllSeries<T, V>(values: T[], mapper: (obj: T) => Promise<V>): Promise<V[]> {
             return promisePoolAsync(1, values, mapper);
         }
 
@@ -193,7 +193,7 @@ namespace pxsim {
                 }, ms);
             });
 
-            return Promise.race([ promise, timeoutPromise ])
+            return Promise.race([promise, timeoutPromise])
                 .then(output => {
                     // clear any dangling timeout
                     if (res) {
@@ -311,6 +311,14 @@ namespace pxsim {
             })
             return v;
         }
+
+        export function sanitizeCssName(name: string): string {
+            let sanitized = name.replace(/[^a-zA-Z0-9-_]/g, '_');
+            if (!/^[a-zA-Z_]/.test(sanitized)) {
+                sanitized = 'cls_' + sanitized;
+            }
+            return sanitized;
+        }
     }
 
     export interface Map<T> {
@@ -575,7 +583,7 @@ namespace pxsim {
 
     class EventHandler {
         private busy = 0;
-        constructor(public handler: RefAction, public flags: number) {}
+        constructor(public handler: RefAction, public flags: number) { }
 
         async runAsync(eventValue: EventIDType, runtime: Runtime, valueToArgs?: EventValueToActionArgs) {
             // The default behavior can technically be configured in codal, but we always set it to queue if busy
@@ -597,9 +605,9 @@ namespace pxsim {
         }
 
         private async runFiberAsync(eventValue: EventIDType, runtime: Runtime, valueToArgs?: EventValueToActionArgs) {
-            this.busy ++;
+            this.busy++;
             await runtime.runFiberAsync(this.handler, ...(valueToArgs ? valueToArgs(eventValue) : [eventValue]));
-            this.busy --;
+            this.busy--;
         }
     }
 
@@ -680,7 +688,7 @@ namespace pxsim {
                 this._handlers = [new EventHandler(a, flags)];
             }
             else {
-                this._addRemoveLog.push({ act: a, log: LogType.UserSet, flags});
+                this._addRemoveLog.push({ act: a, log: LogType.UserSet, flags });
             }
         }
 
diff --git a/pxtsim/simdriver.ts b/pxtsim/simdriver.ts
index 0f8710e2d0f..edd30cac22a 100644
--- a/pxtsim/simdriver.ts
+++ b/pxtsim/simdriver.ts
@@ -393,13 +393,15 @@ namespace pxsim {
                             this.options.simulatorExtensions &&
                             this.options.simulatorExtensions[messageChannel];
 
-                        const startSimulatorExtension = (url: string, permanent: boolean) => {
+                        const startSimulatorExtension = (url: string, permanent: boolean, aspectRatio: number) => {
+                            aspectRatio = aspectRatio || this._runOptions?.aspectRatio || 1.22;
                             let wrapper = this.createFrame(url);
                             this.container.appendChild(wrapper);
                             const messageFrame = wrapper.firstElementChild as HTMLIFrameElement;
                             messageFrame.dataset[FRAME_DATA_MESSAGE_CHANNEL] = messageChannel;
+                            messageFrame.dataset[FRAME_ASPECT_RATIO] = aspectRatio + "";
                             pxsim.U.addClass(wrapper, "simmsg")
-                            pxsim.U.addClass(wrapper, "simmsg" + messageChannel)
+                            pxsim.U.addClass(wrapper, "simmsg" + U.sanitizeCssName(messageChannel))
                             if (permanent)
                                 messageFrame.dataset[PERMANENT] = "true";
                             this.startFrame(messageFrame);
@@ -417,7 +419,7 @@ namespace pxsim {
                                     url.searchParams.set("parentOrigin", encodeURIComponent(this.options.parentOrigin));
                                 if (this.options.userLanguage)
                                     url.searchParams.set("language", encodeURIComponent(this.options.userLanguage));
-                                startSimulatorExtension(url.toString(), simulatorExtension.permanent);
+                                startSimulatorExtension(url.toString(), simulatorExtension.permanent, simulatorExtension.aspectRatio);
                             }
                             // not running the current run, restart
                             else if (messageFrame.dataset['runid'] != this.runId) {
@@ -434,7 +436,7 @@ namespace pxsim {
                                 const url = ((useLocalHost && messageSimulator.localHostUrl) || messageSimulator.url)
                                     .replace("$PARENT_ORIGIN$", encodeURIComponent(this.options.parentOrigin || ""))
                                     .replace("$LANGUAGE$", encodeURIComponent(this.options.userLanguage))
-                                startSimulatorExtension(url, messageSimulator.permanent);
+                                startSimulatorExtension(url, messageSimulator.permanent, messageSimulator.aspectRatio);
                             }
                             // not running the curren run, restart
                             else if (messageFrame.dataset['runid'] != this.runId) {