The answer is neither; it isn't remotely imitating the Lisa's architecture, nor is it a skin over some other previously written JS code. To be clear: this is absolutely NOT a CSS skin.
The only CSS involved is used for the positioning of the canvas element on the page, and for the styling of the temporary UI elements which mimic the Lisa's hardware test screen (and those that display as a fallback if JS is disabled). When the system "boots" all those temporary elements are replaced with a single canvas element on which the UI is displayed.
I should perhaps have been less modest in my original comment in that all of the UI components are coded from scratch. This includes a hierarchical system for displaying objects on the canvas (not dissimilar to the the DOM), the windowing system, a typesetting system, multiple systems for displaying and storing image data, and much more. I've written what I hesitate to call a graphics engine, but I guess that's sort of what it is... it even supports 1-bit blending modes.
This was originally going to be more along the lines of a CSS skin, but I became so deeply frustrated with visual inconsistencies across browser engines and platforms that I ultimately decided to move all the logic I could into JS. The result is what you see now.
Oh, gosh, I didn't mean a CSS skin, just, like, a frontend over custom/modern software with sensible design patterns meant to be sensible on modern hardware. I'm guessing alpha.lisagui.com is the URL; it's pretty slick!
