I have a problem where my CSS is not taking effect (in Chrome), and I think there is some conflict with Twitter Bootstrap.
input.ng-invalid { border-color: r
You can easily do it with ng-class directive:
ng-class
Salt:
http://plnkr.co/edit/RihsxA?p=preview
EDIT
Example with bootstrap 3.3.5 and angular 1.4.2:
http://plnkr.co/edit/5JNCrY8yQmcnA9ysC7Vc?p=preview