You could also look at building a graphical tool-kit (think GTK/QT) that can be used to render windows and their styling.

Finally, you could look at building a window manager with a bunch of applications which look the way you would like it to. This is probably the most achievable goal, but it also offers you the least flexibility with your rendering.

A couple of other points: - I would definitely recommend targeting a non-mac *nix environment. My understanding of the situation is that on Mac and Windows their is little support for external renderers and basically none for custom window managers. - Most people would consider JS is to be a pretty bad language to write one of these systems in. You will be able to use JS as the end user application programming interface if you like (a la Gnome 3), but you will likely have a tough time writing any of the three systems mentioned above in pure JS. HTML and CSS will likely only be useful for the front-end that you expose to the end users.

Finally,

> What would be the shortest/fastest path to me being able to write my own entirely custom UX?

In the field of hobby OS/systems programming the 'shortest and fastest' route is a very bad attitude to take. Building anything in this area takes time and patience, if you skim on either of these you likely won't get far.

I hope this is helpful -- good luck! Sam

You can get a lot smaller than 5mb source on that if you're willing to ditch the X protocol. You'd end up with something like RiscOS, Amiga Workbench, PC GEM, or very early MacOS.

> shortest/fastest path to me being able to write my own entirely custom UX

Take your existing windowing environment and language tooling and ask it for a fullscreen framebuffer. Draw in that. You can be up and running in minutes.

> Preferably something I could do in HTML/CSS/JS and be able to run on a desktop/laptop/phone

.. oh. That requires an entire pile of browser machinery and is very far from 'bare metal'. Fullscreen node-webkit?