Simple question:
Why is it that the summary class can directly be appended to the <span> element but not the ng-classes to the <form> tag?
Is it because the ng-classes are later generated during runtime? What are the guidelines?
CSS:

HTML:

That is not how you use ng-class. Instead do this:
<span class="summary" ng-class="{ 'ng-valid': myForm.$valid, 'ng-invalid': !myForm.$valid }">...</span>
If you want to do it the way you have it do it like this:
<span class="summary {{ myForm.$valid ? 'ng-valid' : 'ng-invalid' }}">...</span>
Next time you need help on stackoverflow, please paste the code instead of images. Would make it a lot easier.
form ng-invalid.ng-dirty compared to span.summary.ng-invalid css selector part.