[webcomponents] ES5 consideration for custom elements (#423)

We think this is okay but I'll note that the new constructor based custom elements API is incompatible with ES5 because there is no mechanism to set new.target in ES5 without use of `Reflect.construct`.

This is probably non-issue though because by the time custom elements API will be implemented by all browsers, `Reflect.construct` would certainly be available everywhere.

This is just FYI.

---
Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/423