The interoperability of Web Component polyfills

At the moment, there are two polyfills for the upcoming Web Components standard (roughly, for widgets): Fortunately, the three ways of writing a Web Component (via the Polymer polyfill, via the X-Tag polyfill or by using the APIs directly) are interoperable. Quoting “Custom Element Interoperability” by the Polymer team:
We’re happy to say that, yes, custom elements of any variety (be they Polymer, X-Tag or vanilla) can all happily coexist.
The blog post explains how that works and provides an example that you can download from GitHub.

Third Web Component polyfill: @b_lionel told me about Bosonic, which is partially based on Polymer and also interoperable.