diff --git a/src/core/VPApp.cpp b/src/core/VPApp.cpp
index 7175dd2..f05a324 100644
--- a/src/core/VPApp.cpp
+++ b/src/core/VPApp.cpp
@@ -36,6 +36,10 @@
 #include <libwinevbs/libwinevbs.h>
 #endif
 
+#ifdef __EMSCRIPTEN__
+#include <emscripten.h>
+#endif
+
 #include "parts/ball.h"
 #include "parts/timer.h"
 #include "parts/flipper.h"
@@ -273,6 +277,34 @@ int VPApp::GetLogicalNumberOfProcessors() const
    return m_logicalNumberOfProcessors;
 }
 
+#ifdef __EMSCRIPTEN__
+// VBScript's MsgBox() must return synchronously (its VARIANT result is used
+// right after the call, in the same script statement) - window.confirm()/
+// alert() are the only browser APIs that block JS execution and return a
+// value synchronously, so they're used here directly rather than routing
+// through any async/JS-callback mechanism the host page might otherwise want
+// to customize the look of (which would require Asyncify to suspend the
+// call, since single-threaded builds have no other way to block for it).
+// type is the raw VBScript "buttons" argument (MB_OK=0, MB_OKCANCEL=1,
+// MB_ABORTRETRYIGNORE=2, MB_YESNOCANCEL=3, MB_YESNO=4, MB_RETRYCANCEL=5).
+// The returned id matches vbOK(1)/vbCancel(2)/vbYes(6)/vbNo(7).
+static int EmscriptenMsgBox(const char* prompt, int type, const char* title)
+{
+   const string message = (title && title[0]) ? (string(title) + "\n\n" + prompt) : string(prompt);
+   switch (type & 0x0F)
+   {
+      case 1: // MB_OKCANCEL
+         return EM_ASM_INT({ return confirm(UTF8ToString($0)) ? 1 : 0; }, message.c_str()) ? 1 /* IDOK */ : 2 /* IDCANCEL */;
+      case 3: // MB_YESNOCANCEL - Cancel isn't distinguishable from No via confirm(), best effort
+      case 4: // MB_YESNO
+         return EM_ASM_INT({ return confirm(UTF8ToString($0)) ? 1 : 0; }, message.c_str()) ? 6 /* IDYES */ : 7 /* IDNO */;
+      default:
+         EM_ASM_({ alert(UTF8ToString($0)); }, message.c_str());
+         return 1; // IDOK
+   }
+}
+#endif
+
 void VPApp::InitInstance()
 {
    std::filesystem::path iniFileName = m_commandLineCustomSettingsFileName;
@@ -325,6 +357,9 @@ void VPApp::InitInstance()
          delete[] buffer;
       }
    };
+#ifdef __EMSCRIPTEN__
+   callbacks.msgbox = &EmscriptenMsgBox;
+#endif
    libwinevbs_init(&callbacks);
 #endif