[jQuery] Re: Same code structure to affect two elements, but one doesn't work

[marr.adam]

I think css precedence has got you down. As I remember it, ID
selectors override class selectors (since id is more specific than
class). Try using border: 3px solid red !important; or maybe the
change the css selector to div.maindiv-hover {  ...  } and see if this
fixes your problems. Look up css precedence for the exact order.