I don't think there is much evidence for any early racism regarding African-Americans, although given his origins it would not be surprising, that said he does tend to wax nostalgic about his youth in a slave-owning town, and to deny any problem in that in his various writings focused on his own youth, so that is the kind of thing that is considered somewhat racism-adjacent in our understanding of such matters, at least.
As far as Asian racism he wrote articles against abuse of Chinese workers by police in San Francisco when he worked there as a journalist.
Right! I forgot about his writings on Indians. Not Twain's finest. I wonder why TFA doesn't mention this, instead of going on an irrelevant tangent about the Confederacy?
I have to say I do think Twain's writings about Indians are racist (so yeah, point taken) but always couched in ambivalence; some of it for example looks like exaggeration to ridicule the poetic "Noble Savage" of Fenimore Cooper and others, which is also bullshit (and unfortunately, lives on to modern age, "Dances With Wolves" et al).
As far as Asian racism he wrote articles against abuse of Chinese workers by police in San Francisco when he worked there as a journalist.
However where Native Americans are concerned he was quite racist https://medium.com/luminasticity/mark-twain-and-the-racist-s...