diff --git a/src/OVGA.cpp b/src/OVGA.cpp
--- a/src/OVGA.cpp
+++ b/src/OVGA.cpp
@@ -30,6 +30,9 @@
 #include <version.h>
 #include <FilePath.h>
 #include <ConfigAdv.h>
+#ifdef __EMSCRIPTEN__
+#include <emscripten.h>
+#endif
 
 DBGLOG_DEFAULT_CHANNEL(Vga);
 
@@ -80,6 +83,14 @@
    init_dpi();
 #endif
 
+#ifdef __EMSCRIPTEN__
+   // In the browser the page scales the canvas and owns full screen: always open the game's native
+   // 800x600 window.
+   config_adv.vga_full_screen = 0;
+   config_adv.vga_full_screen_desktop = 0;
+   config_adv.vga_allow_highdpi = 0;
+#endif
+
    win_grab_forced = 0;
    win_grab_user_mode = 0;
    mouse_mode = MOUSE_INPUT_ABS;
@@ -358,6 +369,17 @@
 {
    SDL_Event event;
 
+#ifdef __EMSCRIPTEN__
+   // The game runs its own busy loops; hand control back to the browser (Asyncify) every ~10 ms so
+   // the canvas is presented and input and audio events are delivered.
+   static double last_yield = 0;
+   if( emscripten_get_now() - last_yield >= 10 )
+   {
+      emscripten_sleep(0);
+      last_yield = emscripten_get_now();
+   }
+#endif
+
    while( SDL_PollEvent(&event) )
    {
       switch (event.type)
@@ -640,6 +662,9 @@
 // mode  1: full screen without display mode change (stretched to desktop)
 void Vga::set_full_screen_mode(int mode)
 {
+#ifdef __EMSCRIPTEN__
+   return;   // the page's own full screen button handles this
+#endif
    int result, mouse_x, mouse_y;
    uint32_t flags = config_adv.vga_full_screen_desktop ?
       SDL_WINDOW_FULLSCREEN_DESKTOP : SDL_WINDOW_FULLSCREEN;