While styling a choice control recently, I have noticed that the CSS class reference defined in [Settings > Formatting > Control CSS class / CSS class] accepts rich-text. This value can be manually typed of course, but if you are working in an IDE and copying in these references for sake of custom development there's a chance your CSS will not work properly due to rich-text markup being carried over unintentionally.
This value for the Control CSS class will pickup custom CSS styles for .customtab:
This value for the Control CSS class will not pickup custom CSS styles for .customtab (notice <b></b> tag carryover):
I'm working with Nintex Forms for Office 365 (App Version : NFO 126.96.36.199) in a Chrome Browser.