feat: add support for customized built-ins to FAST SSR

[chrisdholt]

🙋 Feature Request

Currently FAST SSR supports autonomous custom elements and their API's. The implementation for customized built-ins is different and may necessitate alternative support for the renderer. This issue exists to track adding support for customized built-ins via the is attribute.

[yinonov]

Are there plans for safari to pick up custom built-ins?

[chrisdholt]

Are there plans for safari to pick up custom built-ins?

I’m not aware of any at this point - came up in one of the WC F2F. I have my opinions on it :).

On a positive, the polyfill is quite small, and in testing I saw perf benefits for something like button compared to the autonomous custom element at scale.