Invalid Shadow DOM selector

The selector of a component using ViewEncapsulation.ShadowDom doesn't match the custom element tag name requirements.

In order for a tag name to be considered a valid custom element name, it has to:

• Be in lower case.
• Contain a hyphen.
• Start with a letter (a-z).

Debugging the error

Rename your component's selector so that it matches the requirements.

Before:

@Component({
  selector: 'comp',
  encapsulation: ViewEncapsulation.ShadowDom
…
})

After:

@Component({
  selector: 'app-comp',
  encapsulation: ViewEncapsulation.ShadowDom
…
})