We need a compatibility matrix to show which polyfils are available for each browser. We should also compare our compatibility with Googles Polymer library.
I asked Arron to deprioritize this. Besides his other projects, I also prefer x-tags.org to show more clearly where exactly the library falls in the stack rather than assuming everybody knows and compare based on that knowledge. I'll file/update a bug for this.
x-tag is an independent community project now and tracked in their bug tracker.