There is a debate on whether HTML classes belong in your content. As in, classes that are strictly related to the presentation of that content. Sometimes the use of these classes is unavoidable. A callout paragraph, a pull quote, a carousel in the middle of a post... you'll need classes to style and add functionality to these things.
While you sometimes need them, the less you write them into the actual post content, in my opinion, the better.… Read article “Leverage WordPress Functions to Reduce HTML in your Posts”