Validação usando angulares

validation.html

<div class="large-12 columns">
  <label>Username</label>
    <input  type="text" 
            placeholder="Desired username" 
            name="username" <!-- sá porra aqui tem que tá no teu controller como $scope.username -->
            ng-model="signup.username" 
            ng-minlength=3 
            ng-maxlength=20 
            ensure-unique="username" required />
  <div class="error-container" ng-show="signup_form.username.$dirty && signup_form.username.$invalid">
    <small class="error" ng-show="signup_form.username.$error.required">Please input a username</small>
    <small class="error" ng-show="signup_form.username.$error.minlength">Your username is required to be at least 3 characters</small>
    <small class="error" ng-show="signup_form.username.$error.maxlength">Your username cannot be longer than 20 characters</small>
    <small class="error" ng-show="signup_form.username.$error.unique">That username is taken, please try another</small>
  </div>
</div>