--- a/src/App.cpp
+++ b/src/App.cpp
@@ -24,6 +24,9 @@
 #include <fstream>
 #include <cmath>
 #include <cstdlib>
+#ifdef PLATFORM_WEB
+#include <emscripten/emscripten.h>
+#endif
 #include <memory>
 #include <vector>
 
@@ -34,6 +37,16 @@
 
 void App::openWindow()
 {
+#ifdef PLATFORM_WEB
+    // In the browser (fredygames.com build): the canvas fills the page and follows its size, nothing about monitors,
+    // no fullscreen at the start, and the browser paces the frames (WindowShouldClose waits for the next one).
+    SetConfigFlags( FLAG_WINDOW_RESIZABLE );
+    InitWindow( EM_ASM_INT( { return window.innerWidth; } ), EM_ASM_INT( { return window.innerHeight; } ), "KIP KEPLER" );
+    SetExitKey( 0 );
+    HideCursor();
+    m_screen.init();
+    return;
+#endif
     bool deck = getenv( "SteamDeck" ) && atoi( getenv( "SteamDeck" ) ) == 1;
     // Pictures and timed test runs stay hidden (and quiet: see run()).
     bool     shot = !m_o.m_shot.empty() || !m_o.m_autotest.empty() || m_o.m_seconds > 0;
@@ -511,7 +524,10 @@
     items.push_back( tr( "menu.episodes" ) );
     items.push_back( tr( "menu.extras" ) );
     items.push_back( tr( "menu.options" ) );
+#ifndef PLATFORM_WEB
+    // A web page has no window to close.
     items.push_back( tr( "menu.quit" ) );
+#endif
     return items;
 }
 
--- a/src/Checker.cpp
+++ b/src/Checker.cpp
@@ -498,7 +498,11 @@
             }
         }
     };
+#ifdef __EMSCRIPTEN__
+    unsigned threads = 1;   // no threads in the web build
+#else
     unsigned threads = std::clamp( std::thread::hardware_concurrency() / 2, 1u, 8u );
+#endif
     std::vector<Search> workers( threads, S );
 
     const size_t MaxNodes = 150000;
--- a/src/Editor.cpp
+++ b/src/Editor.cpp
@@ -1114,9 +1114,15 @@
             m_reachOut = std::move( reach );
             m_checked = true;
         };
+#ifdef __EMSCRIPTEN__
+        // No threads in the web build: the check runs at once.
+        job();
+        (void)wait;
+#else
         if ( wait ) job();
         else
             m_thread = std::thread( job );
+#endif
         pollCheck();
     }
 
--- a/src/Rumble.cpp
+++ b/src/Rumble.cpp
@@ -14,6 +14,8 @@
   #define WIN32_LEAN_AND_MEAN
   #define NOMINMAX
   #include <windows.h>
+#elif defined( __EMSCRIPTEN__ )
+  // The web build: no motors (the browser's pad API is left alone).
 #else
   #include <dirent.h>
   #include <fcntl.h>
@@ -92,6 +94,10 @@
         }
 }
 
+#elif defined( __EMSCRIPTEN__ )
+
+void send( float, float ) {}
+
 #else
 
 int  g_fd = -1;       ///< the device with force feedback
--- a/src/Screen.cpp
+++ b/src/Screen.cpp
@@ -16,8 +16,15 @@
 namespace
 {
 
+// The web build runs on WebGL 2 (OpenGL ES 3.0): the same shaders under the GLSL ES header.
+#ifdef PLATFORM_WEB
+#define KEPLER_GLSL "#version 300 es\nprecision mediump float;"
+#else
+#define KEPLER_GLSL "#version 330"
+#endif
+
 // The light map holds half the light, so lamps can brighten up to twice (the lit shader doubles it again).
-const char *LitFs = R"(#version 330
+const char *LitFs = KEPLER_GLSL R"(
 in vec2 fragTexCoord;
 in vec4 fragColor;
 uniform sampler2D texture0;
@@ -31,7 +38,7 @@
 }
 )";
 
-const char *BlurFs = R"(#version 330
+const char *BlurFs = KEPLER_GLSL R"(
 in vec2 fragTexCoord;
 in vec4 fragColor;
 uniform sampler2D texture0;
--- a/third_party/raylib/src/platforms/rcore_web.c
+++ b/third_party/raylib/src/platforms/rcore_web.c
@@ -151,6 +151,11 @@
 //----------------------------------------------------------------------------------
 
 // Check if application should close
+// fredygames.com: wait for the browser's next animation frame instead of a fixed 16 ms sleep (Asyncify)
+EM_ASYNC_JS(void, WaitNextAnimationFrame, (void), {
+    await new Promise(resolve => requestAnimationFrame(() => resolve()));
+});
+
 bool WindowShouldClose(void)
 {
     // Emterpreter-Async required to run sync code
@@ -158,7 +163,7 @@
     // By default, this function is never called on a web-ready raylib example because we encapsulate
     // frame code in a UpdateDrawFrame() function, to allow browser manage execution asynchronously
     // but now emscripten allows sync code to be executed in an interpreted way, using emterpreter!
-    emscripten_sleep(16);
+    WaitNextAnimationFrame();
     return false;
 }
 
--- a/third_party/raylib/src/rcore.c
+++ b/third_party/raylib/src/rcore.c
@@ -1716,6 +1716,12 @@
 {
     if (seconds < 0) return;    // Security check
 
+#if defined(PLATFORM_WEB)
+    // fredygames.com: the browser paces the frames (WindowShouldClose waits for requestAnimationFrame); nanosleep()
+    // would only spin the main thread here.
+    return;
+#endif
+
 #if defined(SUPPORT_BUSY_WAIT_LOOP) || defined(SUPPORT_PARTIALBUSY_WAIT_LOOP)
     double destinationTime = GetTime() + seconds;
 #endif