angular-cn/aio/content/errors/NG2009.md

@name Invalid Shadow DOM selector @category compiler @shortDescription Component selector does not match shadow DOM requirements

@description 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 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
  ...
})