diff --git a/CMakeLists.txt b/CMakeLists.txt
index a4dec6d..9ede742 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -288,7 +288,7 @@ if(WARN_VARIABLE_LENGTH_ARRAY_AVAILABLE)
 endif(WARN_VARIABLE_LENGTH_ARRAY_AVAILABLE)
 
 if(EMSCRIPTEN)
-	set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fno-rtti -fno-exceptions -O3 -s USE_SDL=2 -s USE_ZLIB=1")
+	set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -DEMSCRIPTEN -ffile-prefix-map=${CMAKE_SOURCE_DIR}=hatari -ffile-prefix-map=${CMAKE_BINARY_DIR}=build -fno-rtti -fno-exceptions -O3 -s USE_SDL=2 -s USE_ZLIB=1")
 	set(CMAKE_EXECUTABLE_SUFFIX ".html")
 endif()
 
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 540c97e..a022d00 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -12,7 +12,7 @@ set(SOURCES
 	vdi.c inffile.c video.c wavFormat.c xbios.c ymFormat.c lilo.c)
 
 if(EMSCRIPTEN)
-	set(SOURCES ${SOURCES} emscripten.js emscripten_shell.html)
+	set(SOURCES ${SOURCES} lcweb.c emscripten.js emscripten_shell.html)
 endif()
 
 # Disk image code is shared with the hmsa tool, so we put it into a library:
@@ -202,14 +202,7 @@ elseif(EMSCRIPTEN)
 		${APP_NAME}
 		PROPERTIES
 		LINK_FLAGS
-		"-O3 \
--fno-exceptions \
--s USE_SDL=2 -s USE_ZLIB=1 -s INITIAL_MEMORY=262144000 -s ASYNCIFY -s -s ASSERTIONS=0 \
--s EXPORTED_RUNTIME_METHODS='[\"ccall\",\"cwrap\"]' \
--s EXPORTED_FUNCTIONS=\"['_main','_Reset_Warm','_Reset_Cold','_Statusbar_UpdateInfo','_Main_UnPauseEmulation','_Floppy_InsertDiskIntoDrive','_Floppy_SetDiskFileName','_Floppy_EjectDiskFromDrive','_Configuration_ChangeMemory','_Configuration_ChangeSystem','_Configuration_ChangeTos','_Configuration_ChangeUseHardDiskDirectories','_Configuration_ChangeFastForward','_Configuration_Apply','_IoMem_UnInit','_IoMem_Init']\" \
---preload-file ${CMAKE_BINARY_DIR}/files/@/share/hatari/ \
---pre-js ${CMAKE_SOURCE_DIR}/src/emscripten.js \
---shell-file ${CMAKE_SOURCE_DIR}/src/emscripten_shell.html"
+		"-O3 -fno-exceptions -sUSE_SDL=2 -sUSE_ZLIB=1 -sINITIAL_MEMORY=67108864 -sASYNCIFY -sASSERTIONS=0 -sEXPORTED_RUNTIME_METHODS=ccall,cwrap,FS,HEAPU8,ENV -sMODULARIZE -sEXPORT_ES6 -sEXPORT_NAME=createHatari -sENVIRONMENT=web -sINCOMING_MODULE_JS_API=arguments,canvas,print,printErr,preRun,postRun,preInit,locateFile,wasmBinary,onRuntimeInitialized,onAbort,setStatus,monitorRunDependencies -sEXPORTED_FUNCTIONS=_main,_Reset_Warm,_Reset_Cold,_Main_UnPauseEmulation,_Floppy_InsertDiskIntoDrive,_Floppy_SetDiskFileName,_Floppy_EjectDiskFromDrive,_Configuration_Apply,_malloc,_free,_lcweb_set_input,_lcweb_linea,_lcweb_ram,_lcweb_ram_size,_lcweb_screen_base,_lcweb_palette,_lcweb_shifter_mode,_lcweb_ram_hash --preload-file ${CMAKE_BINARY_DIR}/files/@/share/hatari/ --pre-js ${CMAKE_SOURCE_DIR}/src/emscripten.js --shell-file ${CMAKE_SOURCE_DIR}/src/emscripten_shell.html ${LC_EXTRA_LINK}"
 	)
 # Other targets, use default sources
 else()
diff --git a/src/cpu/CMakeLists.txt b/src/cpu/CMakeLists.txt
index e91d6a7..53df8ed 100644
--- a/src/cpu/CMakeLists.txt
+++ b/src/cpu/CMakeLists.txt
@@ -18,30 +18,33 @@ set(WINUAE_SRCS cpudefs.c cpummu.c cpummu030.c debug.c disasm.c newcpu_common.c
 # once for cross compiling (with calling the host cc directly) and once
 # for native compiling so that the rules also work for non-Unix environments...
 if(CMAKE_CROSSCOMPILING)
+	if(NOT HOST_CC)
+		set(HOST_CC cc)
+	endif()
 
-	add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/build68k
-		COMMAND cc -I${CMAKE_CURRENT_SOURCE_DIR}
+	add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/build68k${HOST_EXE}
+		COMMAND ${HOST_CC} -I${CMAKE_CURRENT_SOURCE_DIR}
 			   ${CMAKE_CURRENT_SOURCE_DIR}/build68k.c
 			   ${CMAKE_CURRENT_SOURCE_DIR}/writelog.c
-			   -o ${CMAKE_CURRENT_BINARY_DIR}/build68k
+			   -o ${CMAKE_CURRENT_BINARY_DIR}/build68k${HOST_EXE}
 		DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/build68k.c)
 
 	add_custom_command(OUTPUT cpudefs.c
-		COMMAND ./build68k < ${CMAKE_CURRENT_SOURCE_DIR}/table68k >cpudefs.c
+		COMMAND ${CMAKE_CURRENT_BINARY_DIR}/build68k${HOST_EXE} < ${CMAKE_CURRENT_SOURCE_DIR}/table68k >cpudefs.c
 		DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/table68k
-			${CMAKE_CURRENT_BINARY_DIR}/build68k)
+			${CMAKE_CURRENT_BINARY_DIR}/build68k${HOST_EXE})
 
-	add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/gencpu
-		COMMAND cc -I${CMAKE_CURRENT_SOURCE_DIR}
+	add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/gencpu${HOST_EXE}
+		COMMAND ${HOST_CC} -I${CMAKE_CURRENT_SOURCE_DIR}
 			   cpudefs.c ${CMAKE_CURRENT_SOURCE_DIR}/gencpu.c
 			   ${CMAKE_CURRENT_SOURCE_DIR}/readcpu.c
-			   -o ${CMAKE_CURRENT_BINARY_DIR}/gencpu
+			   -o ${CMAKE_CURRENT_BINARY_DIR}/gencpu${HOST_EXE}
 		DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/gencpu.c
 			${CMAKE_CURRENT_SOURCE_DIR}/readcpu.c cpudefs.c)
 
 	add_custom_command(OUTPUT ${CPUEMU_SRCS}
-		COMMAND ${CMAKE_CURRENT_BINARY_DIR}/gencpu
-		DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/gencpu)
+		COMMAND ${CMAKE_CURRENT_BINARY_DIR}/gencpu${HOST_EXE}
+		DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/gencpu${HOST_EXE})
 
 else()	# Rules for normal build follow
 
diff --git a/src/emscripten.js b/src/emscripten.js
index 57a5860..8b12cfc 100644
--- a/src/emscripten.js
+++ b/src/emscripten.js
@@ -1,4 +1,5 @@
-Module['arguments']=[
-  '--desktop','false',
-  '-d','/share/hatari/fs/',
-  '--machine','ste'];
+Module['arguments'] = Module['arguments'] || [
+  '--desktop', 'false', '--confirm-quit', 'false', '--statusbar', 'false', '--drive-led', 'false',
+  '--machine', 'st', '--memsize', '1024', '--tos', '/share/hatari/tos.img',
+  '--monitor', 'rgb', '--sound', 'off',
+  '-d', '/share/hatari/fs/', '--auto', 'C:\LASRCHES.PRG'];
diff --git a/src/includes/lcweb.h b/src/includes/lcweb.h
new file mode 100644
index 0000000..72610e0
--- /dev/null
+++ b/src/includes/lcweb.h
@@ -0,0 +1,19 @@
+/*
+  Laser Chess web front end glue (Emscripten only).
+*/
+#ifndef HATARI_LCWEB_H
+#define HATARI_LCWEB_H
+
+#include <stdint.h>
+
+extern void LCWeb_Vbl(void);
+extern void lcweb_set_input(int x, int y, int buttons);
+extern uint32_t lcweb_linea(void);
+extern uint8_t *lcweb_ram(void);
+extern uint32_t lcweb_ram_size(void);
+extern uint32_t lcweb_screen_base(void);
+extern int lcweb_palette(int n);
+extern int lcweb_shifter_mode(void);
+extern uint32_t lcweb_ram_hash(void);
+
+#endif
diff --git a/src/lcweb.c b/src/lcweb.c
new file mode 100644
index 0000000..6609b11
--- /dev/null
+++ b/src/lcweb.c
@@ -0,0 +1,141 @@
+/*
+  Laser Chess web front end glue (Emscripten only).
+
+  JavaScript owns the clock: at every emulated VBL the emulator yields to
+  Module.lcVbl(frame), which resolves when the next frame may run (local
+  pacing, or once both online players' inputs for that frame are known).
+  Input is an absolute ST pointer position plus button state; it is turned
+  into relative IKBD mouse packets here, from the GEM cursor position kept
+  in the Line-A variables, so both online peers derive identical packets
+  from identical emulated state.
+*/
+#include "main.h"
+#include "configuration.h"
+#include "stMemory.h"
+#include "ikbd.h"
+#include "ioMem.h"
+#include "video.h"
+#include "lcweb.h"
+#include <emscripten.h>
+
+static int in_x = 160, in_y = 100, in_buttons = 0;	/* wanted pointer state */
+static int cur_buttons = 0;				/* buttons as sent to the ST */
+static int hold_frames = 0;
+static uint32_t linea = 0;
+static uint32_t frame_no = 0;
+
+EM_ASYNC_JS(int, lc_js_vbl, (int frame), {
+	return await Module.lcVbl(frame);
+});
+
+EMSCRIPTEN_KEEPALIVE void lcweb_set_input(int x, int y, int buttons)
+{
+	in_x = x;
+	in_y = y;
+	in_buttons = buttons;
+}
+
+EMSCRIPTEN_KEEPALIVE uint32_t lcweb_linea(void)
+{
+	return linea;
+}
+
+EMSCRIPTEN_KEEPALIVE uint8_t *lcweb_ram(void)
+{
+	return STRam;
+}
+
+EMSCRIPTEN_KEEPALIVE uint32_t lcweb_ram_size(void)
+{
+	return STRamEnd;
+}
+
+EMSCRIPTEN_KEEPALIVE uint32_t lcweb_screen_base(void)
+{
+	return Video_GetScreenBaseAddr();
+}
+
+/* ST palette register n as 0x0RGB (STE 4-bit nibbles) */
+EMSCRIPTEN_KEEPALIVE int lcweb_palette(int n)
+{
+	return IoMem_ReadWord(0xff8240 + n * 2);
+}
+
+EMSCRIPTEN_KEEPALIVE int lcweb_shifter_mode(void)
+{
+	return IoMem_ReadByte(0xff8260) & 3;
+}
+
+/* FNV-1a over ST RAM, for desync checks between online peers */
+EMSCRIPTEN_KEEPALIVE uint32_t lcweb_ram_hash(void)
+{
+	uint32_t h = 2166136261u;
+	for (uint32_t i = 0; i < STRamEnd; i++)
+	{
+		h ^= STRam[i];
+		h *= 16777619u;
+	}
+	return h;
+}
+
+/* The Line-A variable block: PLANES at base, WIDTH at +2, BYTES_LIN at -2,
+ * V_REZ_VT at -4, V_REZ_HZ at -0xc. Look for the ST low-res values. */
+static void find_linea(void)
+{
+	for (uint32_t a = 0x400; a + 4 < STRamEnd && a < 0x40000; a += 2)
+	{
+		if (STMemory_ReadWord(a) == 4 && STMemory_ReadWord(a + 2) == 160
+		    && STMemory_ReadWord(a - 2) == 160 && STMemory_ReadWord(a - 4) == 200
+		    && STMemory_ReadWord(a - 0xc) == 320)
+		{
+			linea = a;
+			return;
+		}
+	}
+}
+
+static int clamp(int v, int lo, int hi)
+{
+	return v < lo ? lo : v > hi ? hi : v;
+}
+
+static void apply_input(void)
+{
+	int dx = 0, dy = 0, at = 1;
+
+	if (!linea && frame_no > 50 && (frame_no % 25) == 0)
+		find_linea();
+	if (linea)
+	{
+		int cx = (int16_t)STMemory_ReadWord(linea - 0x25a);
+		int cy = (int16_t)STMemory_ReadWord(linea - 0x258);
+		dx = clamp(in_x - cx, -60, 60);
+		dy = clamp(in_y - cy, -60, 60);
+		at = (dx == 0 && dy == 0);
+	}
+	KeyboardProcessor.Mouse.dx += dx;
+	KeyboardProcessor.Mouse.dy += dy;
+
+	/* A button change waits until the cursor has arrived (at most 10
+	 * frames), so a click lands where it was made. */
+	if (in_buttons != cur_buttons)
+	{
+		if (at || ++hold_frames > 10)
+		{
+			cur_buttons = in_buttons;
+			hold_frames = 0;
+		}
+	}
+	else
+		hold_frames = 0;
+	Keyboard.bLButtonDown = (Keyboard.bLButtonDown & ~BUTTON_MOUSE) | ((cur_buttons & 1) ? BUTTON_MOUSE : 0);
+	Keyboard.bRButtonDown = (Keyboard.bRButtonDown & ~BUTTON_MOUSE) | ((cur_buttons & 2) ? BUTTON_MOUSE : 0);
+}
+
+/* Called from Main_WaitOnVbl in place of the host-clock wait. */
+void LCWeb_Vbl(void)
+{
+	frame_no++;
+	lc_js_vbl((int)frame_no);
+	apply_input();
+}
diff --git a/src/main.c b/src/main.c
index dacf9c3..d59b788 100644
--- a/src/main.c
+++ b/src/main.c
@@ -76,6 +76,7 @@ const char Main_fileid[] = "Hatari main.c";
 
 #ifdef EMSCRIPTEN
 #include "emscripten.h"
+#include "lcweb.h"
 #endif
 
 bool bQuitProgram = false;                /* Flag to quit program cleanly */
@@ -355,6 +356,12 @@ void Main_WaitOnVbl(void)
 		exit(0);
 	}
 
+#ifdef EMSCRIPTEN
+	/* Laser Chess web: JavaScript paces frames and feeds input */
+	LCWeb_Vbl();
+	return;
+#endif
+
 //	FrameDuration_micro = (int64_t) ( 1000000.0 / nScreenRefreshRate + 0.5 );	/* round to closest integer */
 	FrameDuration_micro = ClocksTimings_GetVBLDuration_micro ( ConfigureParams.System.nMachineType , nScreenRefreshRate );
 	FrameDuration_micro *= nVBLSlowdown;
@@ -1022,7 +1029,12 @@ void Main_ErrorExit(const char *msg1, const char *msg2, int errval)
 int main(int argc, char *argv[])
 {
 	/* Generate random seed */
+#ifdef EMSCRIPTEN
+	/* deterministic for online lockstep: the seed comes from the page */
+	Hatari_srand(EM_ASM_INT({ return Module.lcSeed | 0; }));
+#else
 	Hatari_srand(time(NULL));
+#endif
 
 	/* Setup for string conversions */
 	Str_Init();