Using Angular markup like `{{hash}}` in an href attribute will
make the link go to the wrong URL if the user clicks it before
Angular has a chance to replace the `{{hash}}` markup with its
value. Until Angular replaces the markup the link will be broken
and will most likely return a 404 error. The `ngHref` directive
solves this problem.
The wrong way to write it:
```html
<a href="http://www.gravatar.com/avatar/{{hash}}">link1</a>
```
The correct way to write it:
```html
<a ng-href="http://www.gravatar.com/avatar/{{hash}}">link1</a>
```