diff --git a/src/azimuth/control/util.c b/src/azimuth/control/util.c
index f0c2650..c09b59a 100644
--- a/src/azimuth/control/util.c
+++ b/src/azimuth/control/util.c
@@ -25,7 +25,7 @@
 #include <stdbool.h>
 #include <stdlib.h>
 
-#if !defined(__WINDOWS__) && !defined(__APPLE__)
+#if !defined(__WINDOWS__) && !defined(__APPLE__) && !defined(__EMSCRIPTEN__)
 #include <string.h>
 #include <sys/stat.h>
 #include <wordexp.h>
@@ -48,7 +48,10 @@
 // If anything fails, this will return a NULL pointer.
 // Free the returned string with SDL_free().
 static char *az_get_app_data_directory(void) {
-#if !defined(__WINDOWS__) && !defined(__APPLE__)
+#if defined(__EMSCRIPTEN__)
+  // The fredygames.com page mounts this folder from IndexedDB before the game starts.
+  return SDL_strdup("/home/web_user/azimuth");
+#elif !defined(__WINDOWS__) && !defined(__APPLE__)
   // First, do tilde-expansion so we get a path in the user's homedir.
   wordexp_t words;
   wordexp("~/.azimuth-game", &words, 0);
diff --git a/src/azimuth/gui/screen.c b/src/azimuth/gui/screen.c
index ba8fda0..f2813e5 100644
--- a/src/azimuth/gui/screen.c
+++ b/src/azimuth/gui/screen.c
@@ -30,6 +30,22 @@
 #include "azimuth/util/misc.h"
 #include "azimuth/util/warning.h"
 
+#ifdef __EMSCRIPTEN__
+#include <emscripten.h>
+
+// Size of the canvas in pixels: the 640x480 screen scaled to fit the screen of the device (the page
+// shrinks or stretches it to the player with CSS), between 1x and 3x.
+EM_JS(double, az_web_canvas_scale, (void), {
+  const s = Math.min(screen.width / 640, screen.height / 480) * (window.devicePixelRatio || 1);
+  return Math.max(1, Math.min(3, s));
+});
+
+// Hands control back to the browser until it paints the next frame.
+EM_ASYNC_JS(void, az_web_wait_frame, (void), {
+  await new Promise((resolve) => requestAnimationFrame(resolve));
+});
+#endif
+
 /*===========================================================================*/
 
 #ifdef __APPLE__
@@ -96,11 +112,21 @@ void az_init_gui(bool fullscreen, bool enable_audio) {
   SDL_GL_SetAttribute(SDL_GL_MULTISAMPLEBUFFERS, 1);
   SDL_GL_SetAttribute(SDL_GL_MULTISAMPLESAMPLES, 2);
   SDL_GetDesktopDisplayMode(0, &display_mode);
+#ifdef __EMSCRIPTEN__
+  // In the browser the canvas has a fixed size and the page handles full screen.
+  (void)display_mode;
+  fullscreen = false;
+  const double scale = az_web_canvas_scale();
+  window = SDL_CreateWindow(
+    "Azimuth", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED,
+    (int)(AZ_SCREEN_WIDTH * scale), (int)(AZ_SCREEN_HEIGHT * scale), SDL_WINDOW_OPENGL);
+#else
   window = SDL_CreateWindow(
     "Azimuth",
     SDL_WINDOWPOS_CENTERED_DISPLAY(0), SDL_WINDOWPOS_CENTERED_DISPLAY(0),
     display_mode.w, display_mode.h,
     SDL_WINDOW_RESIZABLE | SDL_WINDOW_OPENGL | (fullscreen ? SDL_WINDOW_FULLSCREEN : 0));
+#endif
   if (NULL == window) {
     AZ_FATAL("SDL_CreateWindow failed: %s\n", SDL_GetError());
   }
@@ -141,6 +167,11 @@ void az_set_fullscreen(bool fullscreen) {
   int display_index = 0;
   SDL_DisplayMode display_mode = {0};
   assert(sdl_initialized);
+#ifdef __EMSCRIPTEN__
+  // Ctrl-F and the preference do nothing here; the page has its own full-screen button.
+  if (display_initialized) return;
+  fullscreen = false;
+#endif
   if (display_initialized && fullscreen == currently_fullscreen) return;
   currently_fullscreen = fullscreen;
   az_pause_all_audio();
@@ -151,6 +182,7 @@ void az_set_fullscreen(bool fullscreen) {
     SDL_GetMouseState(&x, &y);
   }
 
+#ifndef __EMSCRIPTEN__
   display_index = SDL_GetWindowDisplayIndex(window);
   SDL_GetDesktopDisplayMode(display_index, &display_mode);
   SDL_SetWindowDisplayMode(window, &display_mode);
@@ -166,6 +198,9 @@ void az_set_fullscreen(bool fullscreen) {
   if (display_initialized) {
     SDL_WarpMouseInWindow(window, x, y);
   }
+#else
+  (void)display_index; (void)display_mode; (void)x; (void)y;
+#endif
 
   // Turn off the depth buffer:
   glDisable(GL_DEPTH_TEST);
@@ -234,9 +269,21 @@ void az_finish_screen_redraw(void) {
   assert(sdl_initialized);
   assert(display_initialized);
   SDL_GL_SwapWindow(window);
+#ifdef __EMSCRIPTEN__
+  // The browser shows the frame once the game yields. Wait for animation frames until the next
+  // 60 Hz tick is near, so the game keeps its speed on 120 Hz and faster displays too.
+  static uint64_t sync_time = 0;
+  uint64_t now;
+  do {
+    az_web_wait_frame();
+    now = az_current_time_nanos();
+  } while (now + 4000000u < sync_time);
+  sync_time = (now > sync_time + 4 * AZ_FRAME_TIME_NANOS ? now : sync_time) + AZ_FRAME_TIME_NANOS;
+#else
   // Synchronize, in case vsync fails to lock us to 60Hz:
   static uint64_t sync_time = 0;
   sync_time = az_sleep_until(sync_time) + AZ_FRAME_TIME_NANOS;
+#endif
 }
 
 void az_gl_scissor(int x, int y, int width, int height) {
diff --git a/src/azimuth/system/resource.c b/src/azimuth/system/resource.c
index f782b02..1ca069c 100644
--- a/src/azimuth/system/resource.c
+++ b/src/azimuth/system/resource.c
@@ -42,8 +42,8 @@ extern const struct resource_entry {
 extern const size_t resource_index_size;
 extern const char _binary_resources_start[];
 
-#if defined(__APPLE__)
-// Use app-bundle resources on macOS
+#if defined(__APPLE__) || defined(__EMSCRIPTEN__)
+// Use app-bundle resources on macOS (and the preloaded files in the browser)
 bool az_system_resource_reader(const char *name, az_reader_t *reader) {
   char *resource_dir = SDL_GetBasePath();
   if (resource_dir == NULL) return false;