## A web frontend for Emacs via CodeMirror

### Motivation

Why a web frontend for Emacs?

1. A web frontend would benefit from being cross-platform in the sense
   that you have one implementation, and now any web browser on any
   platform is capable of providing an interface for you. Currently
   Emacs maintains many codebases for different frontends.
2. Web browsers will support Wayland faster than Emacs will support
   Wayland.
3. You can embed web pages via iframes, which can be used to browse
   documentation, or even creating a web browser within Emacs that can
   display anything a fully fledged browser can.
4. You can embed graphics, animations, canvas, GL and other widgets
   available to a modern web browser, and put this into e.g. a REPL, a
   code editor (e.g. a colour picker in css-mode).
5. Thanks to Electron, you can produce a native "app" from such a web
   interface, that would look no less first-class than graphical Emacs
   itself.
6. Working over a network (e.g. websockets) would allow you to "live
   stream" your Emacs to other people who could view it trivially with
   a web browser (no installation or setup neccessary).
7. It would serve as a nice proof-of-concept/fact-finding project in
   order to consider more ideal implementations, such as an SDL-based
   Cairo/OpenGL-based implementation of Emacs.

### Emacs background

Emacs is a Lisp engine with built-in support for buffers and
windowing. It has frontends: a terminal frontend, a Gtk & X.org based
frontend (capable of displaying more colours and font sizes and
styles), and also Cocoa and Windows frontends.

### Web frontend

To provide a web frontend for Emacs, you would need a web page to
connect to Emacs via a websocket (for which there's a library in Emacs
Lisp) which would:

1. Emacs side: Receive incoming keyboard (text typing and commands)
   and mouse events (clicking, selecting, scrolling) from the client,
   and run them.
2. Web side: Receive updates to the windows and buffers, replacing
   parts of the buffer that changed along with text properties,
   creating and destroying overlays, and send keyboard & mouse events
   to the server.

There would need to be special handling for window management, and for
the minibuffer which is always visible, as well as the modeline and
"fringe" (which is the left-hand-side bar sometimes used by flycheck
and things like that).

### CodeMirror as a frontend

To produce a frontend, you hypothetically only need a few
things. These things are provided by CodeMirror:

* `replaceRegion` - can be used to update changed text coming from Emacs,
  which lets us only change part of the buffer, rather than rewriting
  the whole document.
* `markText` - can be used to set text properties, which is used for
  highlighting and visibility among other things in Emacs.
* `addOverlay` - can be used to create overlays, just as they are done
  in Emacs.
* `setSelection` - can be used to set the cursor position and the
  selection.
* Capturing keyboard input in a cross-platform manner.
* Capturing scrolling and other mouse input in a cross-platform
  manner.

Font faces would be sent from Emacs to the Web client periodically,
and encoded as CSS rules, with custom face properties specified as
`style` properties in the DOM.

### Conclusions

1. The case for a web frontend for Emacs is compelling.
2. It should not be much work to create a CodeMirror frontend to
   Emacs.