diff --git a/core/src/backend.rs b/core/src/backend.rs
index d7e43c85f1b5..f1d8a36b80e6 100644
--- a/core/src/backend.rs
+++ b/core/src/backend.rs
@@ -5,3 +5,4 @@ pub mod log;
 pub mod navigator;
 pub mod render;
 pub mod storage;
+pub mod ui;
diff --git a/core/src/backend/ui.rs b/core/src/backend/ui.rs
new file mode 100644
index 000000000000..e4432a9475a5
--- /dev/null
+++ b/core/src/backend/ui.rs
@@ -0,0 +1,22 @@
+pub trait UiBackend {
+    fn message(&self, message: &str);
+}
+
+/// UiBackend that does mostly nothing
+pub struct NullUiBackend {}
+
+impl NullUiBackend {
+    pub fn new() -> Self {
+        Self {}
+    }
+}
+
+impl UiBackend for NullUiBackend {
+    fn message(&self, _message: &str) {}
+}
+
+impl Default for NullUiBackend {
+    fn default() -> Self {
+        NullUiBackend::new()
+    }
+}
diff --git a/core/src/player.rs b/core/src/player.rs
index 506fe8399c2d..f461869467c3 100644
--- a/core/src/player.rs
+++ b/core/src/player.rs
@@ -9,7 +9,7 @@ use crate::backend::locale::LocaleBackend;
 use crate::backend::navigator::{NavigatorBackend, RequestOptions};
 use crate::backend::storage::StorageBackend;
 use crate::backend::{
-    audio::AudioBackend, log::LogBackend, render::Letterbox, render::RenderBackend,
+    audio::AudioBackend, log::LogBackend, render::Letterbox, render::RenderBackend, ui::UiBackend,
 };
 use crate::context::{ActionQueue, ActionType, RenderContext, UpdateContext};
 use crate::display_object::{EditText, MorphShape, MovieClip};
@@ -23,7 +23,7 @@ use crate::prelude::*;
 use crate::property_map::PropertyMap;
 use crate::tag_utils::SwfMovie;
 use crate::transform::TransformStack;
-use crate::vminterface::Instantiator;
+use crate::vminterface::{AvmType, Instantiator};
 use enumset::EnumSet;
 use gc_arena::{make_arena, ArenaParameters, Collect, GcCell};
 use instant::Instant;
@@ -134,6 +134,7 @@ type Input = Box<dyn InputBackend>;
 type Storage = Box<dyn StorageBackend>;
 type Locale = Box<dyn LocaleBackend>;
 type Log = Box<dyn LogBackend>;
+type UI = Box<dyn UiBackend>;
 
 pub struct Player {
     /// The version of the player we're emulating.
@@ -159,6 +160,7 @@ pub struct Player {
     input: Input,
     locale: Locale,
     log: Log,
+    pub user_interface: UI,
     transform_stack: TransformStack,
     view_matrix: Matrix,
     inverse_view_matrix: Matrix,
@@ -205,6 +207,7 @@ pub struct Player {
     self_reference: Option<Weak<Mutex<Self>>>,
 }
 
+#[allow(clippy::too_many_arguments)]
 impl Player {
     pub fn new(
         renderer: Renderer,
@@ -214,6 +217,7 @@ impl Player {
         storage: Storage,
         locale: Locale,
         log: Log,
+        user_interface: UI,
     ) -> Result<Arc<Mutex<Self>>, Error> {
         let fake_movie = Arc::new(SwfMovie::empty(NEWEST_PLAYER_VERSION));
         let movie_width = 550;
@@ -280,6 +284,7 @@ impl Player {
             input,
             locale,
             log,
+            user_interface,
             self_reference: None,
             system: SystemProperties::default(),
             instance_counter: 0,
@@ -305,10 +310,10 @@ impl Player {
 
         player.build_matrices();
         player.audio.set_frame_rate(frame_rate);
-
         let player_box = Arc::new(Mutex::new(player));
         let mut player_lock = player_box.lock().unwrap();
         player_lock.self_reference = Some(Arc::downgrade(&player_box));
+
         std::mem::drop(player_lock);
 
         Ok(player_box)
@@ -624,7 +629,7 @@ impl Player {
             });
         }
 
-        // Propagte clip events.
+        // Propagate clip events.
         self.mutate_with_update_context(|context| {
             let (clip_event, listener) = match event {
                 PlayerEvent::KeyDown { .. } => {
@@ -799,6 +804,7 @@ impl Player {
     /// This should only be called once. Further movie loads should preload the
     /// specific `MovieClip` referenced.
     fn preload(&mut self) {
+        let mut is_action_script_3 = false;
         self.mutate_with_update_context(|context| {
             let mut morph_shapes = fnv::FnvHashMap::default();
             let root = *context.levels.get(&0).expect("root level");
@@ -806,15 +812,20 @@ impl Player {
                 .unwrap()
                 .preload(context, &mut morph_shapes);
 
+            let lib = context
+                .library
+                .library_for_movie_mut(root.as_movie_clip().unwrap().movie().unwrap());
+
+            is_action_script_3 = lib.avm_type() == AvmType::Avm2;
             // Finalize morph shapes.
             for (id, static_data) in morph_shapes {
                 let morph_shape = MorphShape::new(context.gc_context, static_data);
-                context
-                    .library
-                    .library_for_movie_mut(root.as_movie_clip().unwrap().movie().unwrap())
-                    .register_character(id, crate::character::Character::MorphShape(morph_shape));
+                lib.register_character(id, crate::character::Character::MorphShape(morph_shape));
             }
         });
+        if is_action_script_3 {
+            self.user_interface.message("This SWF contains ActionScript 3 which is not yet supported by Ruffle. The movie may not work as intended.");
+        }
     }
 
     pub fn run_frame(&mut self) {
diff --git a/core/tests/regression_tests.rs b/core/tests/regression_tests.rs
index f8f9744d9d2f..5bf844be7f2f 100644
--- a/core/tests/regression_tests.rs
+++ b/core/tests/regression_tests.rs
@@ -7,6 +7,7 @@ use ruffle_core::backend::locale::NullLocaleBackend;
 use ruffle_core::backend::log::LogBackend;
 use ruffle_core::backend::navigator::{NullExecutor, NullNavigatorBackend};
 use ruffle_core::backend::storage::MemoryStorageBackend;
+use ruffle_core::backend::ui::NullUiBackend;
 use ruffle_core::backend::{
     audio::NullAudioBackend, input::NullInputBackend, render::NullRenderer,
 };
@@ -694,6 +695,7 @@ fn run_swf(
         Box::new(MemoryStorageBackend::default()),
         Box::new(NullLocaleBackend::new()),
         Box::new(TestLogBackend::new(trace_output.clone())),
+        Box::new(NullUiBackend::new()),
     )?;
     player.lock().unwrap().set_root_movie(Arc::new(movie));
     player
diff --git a/desktop/src/main.rs b/desktop/src/main.rs
index 58ade25844e9..c04963871852 100644
--- a/desktop/src/main.rs
+++ b/desktop/src/main.rs
@@ -8,6 +8,7 @@ mod locale;
 mod navigator;
 mod storage;
 mod task;
+mod ui;
 
 use crate::custom_event::RuffleEvent;
 use crate::executor::GlutinAsyncExecutor;
@@ -215,6 +216,7 @@ fn run_player(opt: Opt) -> Result<(), Box<dyn std::error::Error>> {
     )); //TODO: actually implement this backend type
     let input = Box::new(input::WinitInputBackend::new(window.clone()));
     let storage = Box::new(DiskStorageBackend::new());
+    let user_interface = Box::new(ui::DesktopUiBackend::new());
     let locale = Box::new(locale::DesktopLocaleBackend::new());
     let player = Player::new(
         renderer,
@@ -224,6 +226,7 @@ fn run_player(opt: Opt) -> Result<(), Box<dyn std::error::Error>> {
         storage,
         locale,
         Box::new(NullLogBackend::new()),
+        user_interface,
     )?;
     player.lock().unwrap().set_root_movie(Arc::new(movie));
     player.lock().unwrap().set_is_playing(true); // Desktop player will auto-play.
diff --git a/desktop/src/ui.rs b/desktop/src/ui.rs
new file mode 100644
index 000000000000..b90b9f7a6b59
--- /dev/null
+++ b/desktop/src/ui.rs
@@ -0,0 +1,23 @@
+use tinyfiledialogs::{message_box_ok, MessageBoxIcon};
+
+use ruffle_core::backend::ui::UiBackend;
+
+pub struct DesktopUiBackend {}
+
+impl DesktopUiBackend {
+    pub fn new() -> Self {
+        Self {}
+    }
+}
+
+impl UiBackend for DesktopUiBackend {
+    fn message(&self, message: &str) {
+        message_box_ok("Ruffle", message, MessageBoxIcon::Info)
+    }
+}
+
+impl Default for DesktopUiBackend {
+    fn default() -> Self {
+        DesktopUiBackend::new()
+    }
+}
diff --git a/exporter/src/main.rs b/exporter/src/main.rs
index 2194ddb66595..fed6c1719d5a 100644
--- a/exporter/src/main.rs
+++ b/exporter/src/main.rs
@@ -8,6 +8,7 @@ use ruffle_core::backend::locale::NullLocaleBackend;
 use ruffle_core::backend::log::NullLogBackend;
 use ruffle_core::backend::navigator::NullNavigatorBackend;
 use ruffle_core::backend::storage::MemoryStorageBackend;
+use ruffle_core::backend::ui::NullUiBackend;
 use ruffle_core::tag_utils::SwfMovie;
 use ruffle_core::Player;
 use ruffle_render_wgpu::clap::{GraphicsBackend, PowerPreference};
@@ -111,6 +112,7 @@ fn take_screenshot(
         Box::new(MemoryStorageBackend::default()),
         Box::new(NullLocaleBackend::new()),
         Box::new(NullLogBackend::new()),
+        Box::new(NullUiBackend::new()),
     )?;
 
     player
diff --git a/web/packages/core/src/ruffle-player.ts b/web/packages/core/src/ruffle-player.ts
index 658d0d0ed388..c2d3c6f49ccd 100644
--- a/web/packages/core/src/ruffle-player.ts
+++ b/web/packages/core/src/ruffle-player.ts
@@ -790,6 +790,25 @@ export class RufflePlayer extends HTMLElement {
         }
     }
 
+    displayMessage(message: string): void {
+        // Show a dismissible message in front of the player
+        const div = document.createElement("div");
+        div.id = "message_overlay";
+        div.innerHTML = `<div class="message">
+            <div>
+                <p>${message}</p>
+            </div>
+            <div>
+                <button id="continue-btn">continue</button>
+            </div>`;
+        this.container.prepend(div);
+        (<HTMLButtonElement>(
+            this.container.querySelector("#continue-btn")
+        )).onclick = () => {
+            div.remove();
+        };
+    }
+
     protected debugPlayerInfo(): string {
         return `Allows script access: ${this.allowScriptAccess}\n`;
     }
diff --git a/web/packages/core/src/shadow-template.ts b/web/packages/core/src/shadow-template.ts
index 86bfce09d3dc..1391fa8a6994 100644
--- a/web/packages/core/src/shadow-template.ts
+++ b/web/packages/core/src/shadow-template.ts
@@ -25,7 +25,8 @@ ruffleShadowTemplate.innerHTML = `
         #play_button,
         #unmute_overlay,
         #unmute_overlay .background,
-        #panic {
+        #panic,
+        #message_overlay {
             width: inherit;
             height: inherit;
         }
@@ -122,6 +123,39 @@ ruffleShadowTemplate.innerHTML = `
             padding: 10px 50px;
         }
 
+        #message_overlay {
+            position: absolute;
+            background-color: #37528C;
+            color: #FFAD33;
+            opacity: 1.0;
+            z-index: 2;
+            text-align: center;
+        }
+        
+        #message_overlay .message {
+            position: absolute;
+            top: 50%;
+            left: 50%;
+            width: 100%;
+            padding: 20px;
+            transform: translate(-50%, -50%);
+        }
+        
+        #continue-btn {
+             cursor: pointer;
+             background-color: #37528C;
+             color: #FFAD33;
+             border: 2px solid #FFAD33;
+             font-weight: bold;
+             font-size: 20px;
+             border-radius: 20px;
+             padding: 10px;
+        }
+        
+        #continue-btn:hover {
+            background-color: rgba(255, 255, 255, 0.3);
+        }
+
         #right_click_menu {
             color: #FFAD33;
             background-color: #37528c;
diff --git a/web/src/lib.rs b/web/src/lib.rs
index 07ecc4ec75b1..74571610dfc9 100644
--- a/web/src/lib.rs
+++ b/web/src/lib.rs
@@ -7,6 +7,7 @@ mod locale;
 mod log_adapter;
 mod navigator;
 mod storage;
+mod ui;
 
 use crate::log_adapter::WebLogBackend;
 use crate::storage::LocalStorageBackend;
@@ -96,6 +97,9 @@ extern "C" {
 
     #[wasm_bindgen(method)]
     fn panic(this: &JavascriptPlayer, error: &JsError);
+
+    #[wasm_bindgen(method, js_name = "displayMessage")]
+    fn display_message(this: &JavascriptPlayer, message: &str);
 }
 
 struct JavascriptInterface {
@@ -317,7 +321,7 @@ impl Ruffle {
 
         let trace_observer = Arc::new(RefCell::new(JsValue::UNDEFINED));
         let log = Box::new(WebLogBackend::new(trace_observer.clone()));
-
+        let user_interface = Box::new(ui::WebUiBackend::new(js_player.clone()));
         let core = ruffle_core::Player::new(
             renderer,
             audio,
@@ -326,6 +330,7 @@ impl Ruffle {
             local_storage,
             locale,
             log,
+            user_interface,
         )?;
 
         // Create instance.
diff --git a/web/src/ui.rs b/web/src/ui.rs
new file mode 100644
index 000000000000..47b6c85d5f40
--- /dev/null
+++ b/web/src/ui.rs
@@ -0,0 +1,18 @@
+use super::JavascriptPlayer;
+use ruffle_core::backend::ui::UiBackend;
+
+pub struct WebUiBackend {
+    js_player: JavascriptPlayer,
+}
+
+impl WebUiBackend {
+    pub fn new(js_player: JavascriptPlayer) -> Self {
+        Self { js_player }
+    }
+}
+
+impl UiBackend for WebUiBackend {
+    fn message(&self, message: &str) {
+        self.js_player.display_message(message);
+    }
+}