Cool. XML, by way of HTML was always designed to be layered. As in, a layer for a different use case. At the lowest layer, is the data. Each layer used to decorate the layer below it enables using it for different use cases. CSS is just another decoration to create another layer used for another use case (styling the data for the rendering in a browser use case). While keeping the underlying data still usable for a totally different use case. CSS Zen Garden is such a fantastic showcase of that layering architecture of HTML.

Regarding Javascript applications, I consider it just another case where we're using the design of the web for something it's inventors could never foresee. Well, they did foresee "apps", but I'm just not sure React, Angular, Ember, etc. apps was what they were imagining.