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 …})