In a medium bowl, whisk together the rice vinegar, cider vinegar, dill, chili powder, and kosher salt. Whisk in the olive oil tablespoon by tablespoon until a creamy emulsion forms.
In a large bowl, toss the cabbage, red onion, and celery with the dressing. Add ground black pepper to taste, and additional salt as necessary. Serve immediately. (Note: The slaw is best served the day that it is made; leftovers do not save well. One idea is to only add dressing to the vegetables that you plan to eat.)