<div ng-app="main4">
  <md-content ng-controller="main4Ctrl">
    <h2 class="md-display-1 ng-binding">Главная страница 3</h2>

    <sapn id="firstCase">{{myText | transform}}</sapn> - it worked
    <hr>
    <span id="secondCase" ng-bind-html="myText"/>      - but it not worked
  </md-content>
</div>

angular.module('main4', [])
.controller('main4Ctrl', ['$rootScope', '$scope', '$timeout',
  function($rootScope, $scope, $timeout) {
    $scope.myText = 'qWeRtYuIoP';
  }
]).filter('transform',[function(){
  return function(text){
    return transformTetx = text.toLowerCase();
  }
}]);