This is cool and certainly useful but I have concerns. Forgetting for a moment that this is a shim and imagining that all browsers support this natively, isn't this sort of thing outside the intended purpose of html. Html should describe content not styling and this sort of thing seems ripe for abuse. I thought XML was the markup language for describing custom data. Adding the ability to create your own DOM elements for the purpose of using them as hooks for interactivity and styling seems like something that should be avoided in HTML.
Am I being a too much of a purist or overlooking something here?
You're overlooking that the underlying technology - Web Components - is landing as a standard as we speak in both Firefox and Chrome. The idea that creating custom elements is inherently bad is an urban legend - there is no empirical basis for such a claim. Additionally, you are mixing metaphors, Custom Elements are not meant to describe data, they are meant to create new, active UI components and other useful tags.
The web components spec is precisely what AngularJS has been anticipating since Angular's inception. I believe the idea was (and still may be) to integrate web components once they are officially adopted by the major browsers.
Am I being a too much of a purist or overlooking something here?