Angular: Empty Fiddle

http://angularjs.org/

HTML

<script src="http://code.angularjs.org/angular-1.0.1.js"></script>
<div ng-controller="MyCtrl">
  <form name="myForm">
    <input type="text" name="username" ng-model="username" required>
   <p ng-show="myForm.username.$dirty && myForm.username.$error.required">Username is required.</p>
   <button  ng-disabled="myForm.$invalid">Ok</button>
  </form>
</div>

CSS

input{
  border: 3px solid #aaa;
  outline: none;
}
input.ng-dirty.ng-invalid{
  border: 3px solid red;
}

JavaScript

var myApp = angular.module('myApp',[]);

//myApp.directive('myDirective', function() {});
//myApp.factory('myService', function() {});

function MyCtrl($scope) {
    
}