[scoped-custom-element-registry] Safari Tech Preview 214 breaks polyfill

[sorvell]

Description

On Safari Tech Preview version 214 with the scoped custom elements feature flag on (it seems to be default on), the polyfill generates an exception that causes custom elements not to render Shadow DOM content.

Example

Here is a minimal repro that test this issue.

Note, it also tests #609 which is fixed on Safari TP 214 .

Workaround

Making the customElements property writable before loading the polyfill.

workaround example

Browsers affected

• Chrome
• Firefox
• Edge
• [ X] Safari Tech Preview 214
• IE 11