Platform: docs for multiple applications

doc/platforms-html5.dox

@@ -294,6 +294,64 @@ if(CORRADE_TARGET_EMSCRIPTEN)
 endif()
 @endcode
 
+@section platforms-html5-multiple-apps Multiple applications on one page
+
+@note Support for this feature is currently only tested with
+    @ref Platform::EmscriptenApplication. @ref Platform::Sdl2Application
+    and @ref Platform::GlfwApplication support largely depends on Emscripten's
+    underlying emulation libraries and may vary across Emscripten versions.
+
+Running multiple graphical applications on a single page is possible, but
+requires a few changes. By default, Emscripten populates a global @cb{.js} Module @ce
+object with things like callbacks and the canvas element to render into. This is not
+suitable when using more than one application. Emscripten's `MODULARIZE` option
+creates a script that defines a function with a local @cb{.js} Module @ce object
+instead. This way each application runs inside its own function with a separate
+environment:
+
+@code{.cmake}
+target_link_libraries(my-application PRIVATE "-s MODULARIZE -s EXPORT_NAME=createModule")
+@endcode
+
+Instead of running instantly, the application can then be instantiated
+using the function name set with `EXPORT_NAME`. The function optionally
+takes an object to populate the local @cb{.js} Module @ce.
+
+While `EmscriptenApplication.js` populates the @cb{.js} Module @ce object by default,
+it also defines a function @cb{.js} createMagnumModule @ce that lets you create
+new module objects to pass to the Emscripten module function. This allows you
+to include `EmscriptenApplication.js` only once per page without having to bundle it with
+Emscripten's `--pre-js` option. To override the default values, pass an object to
+@cb{.js} createMagnumModule @ce. This is the easiest way to set the canvas or
+status elements of each application:
+
+@code{.html-jinja}
+<script src="EmscriptenApplication.js"></script>
+<script src="{{ application }}.js"></script>
+<script>
+    const myModule = createMagnumModule({
+        canvas: document.getElementById('my-canvas'),
+        status: document.getElementById('my-status'),
+        statusDescription: document.getElementById('my-status-description')
+    });
+    createModule(myModule).then(function(myModule) {
+        // application loaded and running
+    });
+</script>
+@endcode
+
+If you need to make changes to the @cb{.js} Module @ce object created by
+EmscriptenApplication.js, you can add that code to a file that's loaded
+with `--pre-js` right after `EmscriptenApplication.js`. This is usually
+needed for pointing to elements with custom IDs, like the canvas to
+render into or the elements to display status updates:
+
+@code{.js}
+Module.canvas = document.getElementById('my-canvas');
+Module.status = document.getElementById('my-status');
+Module.statusDescription = document.getElementById('my-status-description');
+@endcode
+
 @section platforms-html5-layout Modifying page style, canvas size and aspect ratio
 
 The `WebApplication.css` file contains a basic style and the additional