Add an Using the web editor page (#4629)

This page describes the current workflow and limitations of the
HTML5 Godot editor.

tutorials/editor/index.rst

@@ -22,6 +22,19 @@ in other sections where appropriate. For example, the :ref:`animation editor
    default_key_mapping
    customizing_editor
 
+Web editor
+----------
+
+Godot offers an HTML5 version of the editor running entirely in your browser. No
+download is required to use it, but it has a certain amount of caveats you
+should be aware of.
+
+.. toctree::
+   :maxdepth: 1
+   :name: toc-web-editor
+
+   using_the_web_editor
+
 For developers
 --------------
 

tutorials/editor/using_the_web_editor.rst

@@ -0,0 +1,121 @@
+.. _doc_using_the_web_editor:
+
+Using the Web editor
+====================
+
+Since Godot 3.2.4, there is a `Web editor <https://editor.godotengine.org/>`__
+you can use to work on new or existing projects.
+
+.. note::
+
+    The web editor is in a preliminary stage. While its feature set may be
+    sufficient for educational purposes, it is currently **not recommended for
+    production work**. See :ref:`doc_using_the_web_editor_limitations` below.
+
+Browser support
+---------------
+
+The Web editor requires support for WebAssembly's SharedArrayBuffer. This
+is in turn required to support threading in the browser. The following desktop
+browsers support WebAssembly threading and can therefore run the web editor:
+
+- Chrome 68 or later
+- Firefox 79 or later
+- Edge 79 or later
+
+Opera and Safari are not supported yet. Safari may work in the future once
+proper threading support is added.
+
+**Mobile browsers are currently not supported.**
+
+The web editor supports both the GLES3 and GLES2 renderers, although GLES2 is
+recommended for better performance and compatibility with old/low-end hardware.
+
+.. note::
+
+    If you use Linux, due to
+    `poor Firefox WebGL performance <https://bugzilla.mozilla.org/show_bug.cgi?id=1010527>`__,
+    it's recommended to use a Chromium-based browser instead of Firefox.
+
+.. _doc_using_the_web_editor_limitations:
+
+Limitations
+-----------
+
+Due to limitations on the Godot or Web platform side, the following features
+are currently missing:
+
+- No C#/Mono support.
+- No GDNative support.
+- No project exporting. As a workaround, you can download the project source
+  using **Project > Tools > Download Project Source** and export it using a
+  `native version of the Godot editor <https://godotengine.org/download>`__.
+- The editor won't warn you when closing the tab with unsaved changes.
+- No lightmap baking support. You can still use existing lightmaps if they were
+  baked with a native version of the Godot editor
+  (e.g. by importing an existing project).
+
+The following features are unlikely to be supported due to inherent limitations
+of the Web platform:
+
+- No support for external script editors.
+- No support for Android one-click deploy.
+
+.. seealso::
+
+    See the
+    `list of open issues on GitHub related to the web editor <https://github.com/godotengine/godot/issues?q=is%3Aopen+is%3Aissue+label%3Aplatform%3Ahtml5+label%3Atopic%3Aeditor>`__ for a list of known bugs.
+
+Importing a project
+-------------------
+
+To import an existing project, the current process is as follows:
+
+- Specify a ZIP file to preload on the HTML5 filesystem using the
+  **Preload project ZIP** input.
+- Run the editor by clicking **Start Godot editor**.
+  The Godot project manager should appear after 10-20 seconds.
+  On slower machines or connections, loading may take up to a minute.
+- In the dialog that appears at the middle of the window, specify a name for
+  the folder to create then click the **Create Folder** button
+  (it doesn't have to match the ZIP archive's name).
+- Click **Install & Edit** and the project will open in the editor.
+
+.. attention::
+
+    It's important to place the project folder somewhere in ``/home/web_user/``.
+    If your project folder is placed outside ``/home/web_user/``, you will
+    lose your project when closing the editor!
+
+    When you follow the steps described above, the project folder will always be
+    located in ``/home/web_user/projects``, keeping it safe.
+
+Editing and running a project
+-----------------------------
+
+Unlike the native version of Godot, the web editor is constrained to a single
+window. Therefore, it cannot open a new window when running the project.
+Instead, when you run the project by clicking the Run button or pressing
+:kbd:`F5`, it will appear to "replace" the editor window.
+
+The web editor offers an alternative way to deal with the editor and game
+windows (which are now "tabs"). You can switch between the **Editor** and
+**Game** tabs using the buttons on the top. You can also close the running game
+or editor by clicking the **×** button next to those tabs.
+
+Where are my project files?
+---------------------------
+
+Due to browser security limitations, the editor will save the project files to
+the browser's IndexedDB storage. This storage isn't accessible as a regular folder
+on your machine, but is abstracted away in a database.
+
+You can download the project files as a ZIP archive by using
+**Project > Tools > Download Project Source**. This can be used to export the
+project using a `native Godot editor <https://godotengine.org/download>`__,
+since exporting from the web editor isn't supported yet.
+
+In the future, it may be possible to use the
+`HTML5 FileSystem API <https://developer.mozilla.org/en-US/docs/Web/API/FileSystem>`__
+to store the project files on the user's filesystem as the native editor would do.
+However, this isn't implemented yet.