Issue with the unpkg line.

[Chaotic]

The ‘^’ from line 3 in the url (next to the version number 0.5.2) needs to be removed.

} from 'https://unpkg.com/@polymer/lit-element@^0.5.2/lit-element.js?module';

becomes

} from 'https://unpkg.com/@polymer/lit-element@0.5.2/lit-element.js?module';

[p1ranha]

It didn't solve the "custom element doesn't exist" error for me :(

[nielstron]

@p1ranha For me, the issue with "custom element doesn't exist" was fixed by allowing scripts from unpkg.com in my browser (yes, a local change, nothing to do with the script)

Check the browser javascript console for Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https://unpkg.com/@polymer/lit-element@%5E0.5.2/lit-element.js?module. (Reason: CORS request did not succeed).. If you see this line, you need to enable cross origin requests.