Not My Canon!

Origins and Portrayal

The last component of this contentious debate has to be the way Hermione was originally imagined and portrayed beyond the physical text of the books.

This has been covered in the comments, but since JKR is white, it is quite typical that, unless otherwise specified, characters are written as the nationality of the writer or of the racial majority from the country in which the novel takes place. Most white authors are equally at fault, and only really go into skin color when describing non-white characters. That doesn't make it right, and perhaps is a lesson we need to take away from this. It's a rather consistent practice in modern literature and may be another sign of white privilege to force that assumption. Leaving skin tone ambiguous could be a start, so the reader is allowed to interpret as they wish, but another smart way to denote race, heritage, or nationality is in the naming of a character.

Cho Chang and the Patil twins have typical Chinese/Korean and Indian surnames, and yet their appearance is never explicitly described. In my Fred and George books, I have a character named Jacen Wu, a relief Beater for the Gryffindor Quidditch team. Authors know that their readers are intelligent enough to imagine the relevant character details.

Since this is a discussion about Hermione, we actually can learn a lot about her race through this line of thinking. First, the book takes place in Britain. Based on average demographics, the implied heritage is Anglo-Saxon, especially considering her name. Doing a search across both fictional and historical figures, persons with the name Hermione are generally British/English. The same can be said of people with the surname Granger. And if they happened to be from different parts of the world, they were predominantly white. I think it's safe to say that when a white author living in Britain provides a name like Hermione Granger (with commonly Anglo-Saxon roots) for one of her characters, the likelihood of her not being white becomes increasingly small.

Then we can take JKR's interviews into account.

We know at the start of the series that Hermione was a Mary Sue (an idealized fictional character of young age or low-rank, who saves the day through unrealistic abilities and is often recognized as an "author insert" or a character of wish-fulfillment). JKR confirmed this in an interview.

Interviewer: "Are you Hermione?"

Rowling: "None of the characters in the books are directly taken from life. Real people did inspire a few of them, but of course, once they are on the page they become something completely different. But, yeah, Hermione is a caricature of what I was when I was 11... a real exaggeration, I wasn't that clever. Hermione is a borderline genius at points, and I hope I wasn't that annoying because I would have deserved strangling; sometimes she is an incredible know-it-all."