Why is Huckleberry Finn controversial?

Could you elaborate on the reasons why Huckleberry Finn, a classic American novel, has sparked controversy over the years? Are there specific themes, language choices, or characterizations that have provoked criticism and debates among readers, educators, and cultural commentators? How does the book's treatment of race, slavery, and societal norms contribute to its controversial status? Additionally, have there been any attempts to revise or censor the novel in response to these criticisms?