the last parameter of XChangeProperty is the number of elements.. and when the element format is 32.. the element is "long" so we have 5 long elements here.

Yes this seems confusing as on mac+linux Long is either 32 or 64bits depending on the architecture, but this is how the X11 protocol is defined. Thus 5 is the correct value for the nelts here.  Not 5 or 10 depending on the architecture.

More info on the confusion https://bugs.freedesktop.org/show_bug.cgi?id=16802

docs/README-emscripten.md

@@ -0,0 +1,33 @@
+Emscripten
+================================================================================
+
+Build:
+
+    $ emconfigure ./configure --host=asmjs-unknown-emscripten --disable-assembly --disable-threads --enable-cpuinfo=false CFLAGS="-O2"
+    $ emmake make
+
+Or with cmake:
+
+    $ emconfigure cmake ..
+    $ make
+
+To build one of the tests:
+
+    $ cd test/
+    $ emcc -O2 --js-opts 0 -g4 testdraw2.c -I../include ../build/.libs/libSDL2.a ../build/libSDL2_test.a -o a.html
+
+Uses GLES2 renderer or software
+
+tests: https://dl.dropboxusercontent.com/u/17360362/SDL2-em/index.html
+
+Some other SDL2 libraries can be easily built (assuming SDL2 is installed somewhere):
+
+SDL_mixer (http://www.libsdl.org/projects/SDL_mixer/)
+
+    $ EMCONFIGURE_JS=1 emconfigure ../configure
+    build as usual...
+
+SDL_gfx (http://cms.ferzkopp.net/index.php/software/13-sdl-gfx):
+
+    $ EMCONFIGURE_JS=1 emconfigure ../configure --disable-mmx
+    build as usual...