How to allow html in return of angular2 pipe?

Use bindings to innerHTML or outerHTML to render already escaped html. For example <span [innerHTML]="someString | toHtmlPipe"></span>. See this plunk:

@Pipe({
  name: 'wrapBold'
})
class WrapBold {
  transform(content) {
    return `<b>${content}</b>`;
  }
}

@Component({
  selector: 'my-app',
  pipes: [WrapBold],
  template: `
    <div>
      Hello <span [outerHTML]="content | wrapBold"></span>
    </div>
  `
})
export class App {
  constructor() {
    this.content="Angular2"
  }
}