ng-app vs. data-ng-app, what is the difference?

From Angularjs Documentation

Angular normalizes an element's tag and attribute name to determine which elements match which directives. We typically refer to directives by their case-sensitive camelCase normalized name (e.g. ngModel). However, since HTML is case-insensitive, we refer to directives in the DOM by lower-case forms, typically using dash-delimited attributes on DOM elements (e.g. ng-model).

The normalization process is as follows:

Strip x- and data- from the front of the element/attributes. Convert the :, -, or _-delimited name to camelCase. Here are some equivalent examples of elements that match ngBind:

based on above statement below all are valid directives

1. ng-bind
2. ng:bind
3. ng_bind
4. data-ng-bind
5. x-ng-bind

Good question. The difference is simple - there is absolutely no difference between the two except that certain HTML5 validators will throw an error on a property like ng-app, but they don't throw an error for anything prefixed with data-, like data-ng-app.

So to answer your question, use data-ng-app if you would like validating your HTML to be a bit easier.

Fun fact: You can also use x-ng-app to the same effect.