Typings (index.d.ts) for the html formatter only include the format method.

export interface Formatter {
  format(delta: Delta, original: any): string;
}

export const formatters: {
  annotated: Formatter;
  console: Formatter;
  html: Formatter;
};

showUnchanged and hideUnchanged should also be present.

Given these are only on the HTML formatter we'll probably need to add a separate type for this.