Custom Elements Everywhere is a site created by Rob Dodson. It displays the results of a set of tests that check JS frameworks that use Custom Elements and Shadow DOM for interoperability issues.
It could look like a report card at first glance, but the description at the top of the site nicely sums up the goal of comparing frameworks:
This project runs a suite of tests against each framework to identify interoperability issues, and highlight potential fixes already implemented in other frameworks. If frameworks agree on how they will communicate with Custom Elements, it makes developers' jobs easier; they can author their elements to meet these expectations.
Nice! Consensus and consistency are exactly what Custom Elements needs in light of the official spec being in working draft and the surge in JS frameworks using them.