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

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.

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

The differences lies in the fact that custom data-*attributes are valid in the HTML5 specification. So if you need your markup to be validated, you should use them rather than the ng attributes.

Short Answer:

ng-model and data-ng-model are same and equivalent!

Why?

reason for: data- prefix HTML5 specification expects any custom attribute to be prefixed by data-. reason for: both ng-model and data-ng-model are same and equivalent.

AngularJS Document - Normalization 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: 1. Strip x- and data- from the front of the element/attributes. 2. Convert the :, -, or_-delimited name to camelCase. For example the following forms are all equivalent and match the ngBind directive:

<div ng-controller="Controller">
  Hello <input ng-model='name'> <hr/>
  <span ng-bind="name"></span> <br/>
  <span ng:bind="name"></span> <br/>
  <span ng_bind="name"></span> <br/>
  <span data-ng-bind="name"></span> <br/>
  <span x-ng-bind="name"></span> <br/>
</div>

You can use data-ng-, instead of ng-, if you want to make your page HTML valid.

if you want to manipulate html or html-fragments on your server before serving it to the browser, you most definitely want to be using data-ng-xxx attributes instead of just ng-xxx attributes.

It makes your html valid, meaning it can be used by html (server based) parsers like domdocument (php) or others. These parsers often fail on not well formed html. Angular normalizes the attribute, but remember, that's on the client, not on the server.