Huckleberry Finn was one of the first American novels written in the vernacular style, meaning that it tried to match the way that people spoke rather than using literary language. This is apparent from the opening sentences of the novel:
You don’t know about me, without you have read a book by the name of ‘The Adventures of Tom Sawyer,’ but that ain’t no matter. The book was made by Mr. Mark Twain, and he told the truth, mainly. There was things which he stretched, but mainly he told the truth.
In these three sentences, Twain shows that he is going to be faithful to Huck’s specific Southern dialect. By making grammatical mistakes like “without you have read” (rather than “having read”), “that ain’t no matter” (instead of “that doesn’t matter”), and “there was things” (rather than “there were things”), Twain cements the style of the novel as informal and colloquial.
In choosing to have Huck’s language match how people in his community actually spoke, Twain’s style is also a realist one, meaning he prioritizes capturing the reality of the pre-Civil War American South rather than painting it with a romantic brush. Because Twain has gone to so much effort to realistically depict his characters' language, readers trust that he is realistically depicting the context and circumstances as well, which allows them to sit with the horrors of slavery (one of Twain’s major goals with the book).
Twain also chooses to have Huck directly address the readers at the beginning of the novel (“without you have read”), another stylistic choice that adds to the playful and self-aware nature of the book.