My mistake

HTML

<div ng-controller="myCtrl">
    <span class="" ng-class="getTheClass()">foo</span>
</div>

CSS

.haba-haba {
   color: red;
}

JavaScript

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

function myCtrl($scope) {
    $scope.getTheClass = function() {
        return 'haba-haba';
    };
}