What about producing custom elements?

While listing which libraries work with custom elements is __extremely__ useful, I'd also like to know which frameworks can produce custom elements. Would this be a useful advanced test?

Adding complexity is that some frameworks have third party libraries that do this, (example: [convert react to web components](https://github.com/bitovi/react-to-webcomponent)), should something like this be included?

If there's support, I'd be interested in helping write the tests, etc. Thanks!