How can I enable spell checking on rich text input fields when using either the Nintex classic or Nintex responsive forms in O365?
I figured it out... the spell checker native to the browser is used for spell checking. As you type inside the field you will see the red underlines show up for those words that do not pass the spell checker.
The built-in browser spell check feature DOES NOT work for responsive designer. I've tested it out with a classic form on the same site (different list) and it works just fine. I've also tested out in Chrome and Edge. Same result for both browsers.