This might be a good place to start:

https://bitbucket.org/dhoskin/9webdraw/src/default/

On Thu, Apr 18, 2019 at 7:36 AM Chris McGee <newton688@gmail.com> wrote:

> Hi All,
>
> I'm looking at creating an alternate filesystem for /dev/draw, /dev/mouse
> and /dev/kbd that hooks up to a web server providing HTML interfaces (e.g.
> canvas) for Plan 9 UI. I've been reading over the manual pages, which are
> quite detailed, which is great, but there are some points of confusion for
> me.
>
> In particular, /dev/draw's interface and documentation keep referring to
> the concept of a "window" indirectly. It seems that in some cases the
> server providing /dev/draw needs to track windows and refresh them. But,
> what defines a window in this protocol? Is every image a window or only
> some of them?
>
> Also, I'm trying to understand how off-screen images, such as fonts are
> loaded. It seems that every image must be associated with a screen and be
> given a position within the screen. So, how do you prevent the image from
> being visible to the user?
>
> Hopefully, if I can understand some of the high-level concepts here then
> the manual page will be all that I need. Does anyone have experience with
> this area or could point me to information that might help clarify it?
>
> My next step will probably be to figure out how libmemdraw does all of
> this on top of a frame buffer.
>
> Thanks,
> Chris
>