Shred cabbage into a large bowl; add shredded carrot and onion. Combine mayonnaise, sugar, vinegar, celery seed. Whisk or shake in a jar. Taste and adjust seasonings, adding salt and pepper to taste, or more mayonnaise and a bit more vinegar if too sweet for your taste. Add dressing to the shredded cabbage mixture until well moistened. Refrigerate to chill thoroughly.