Emails are made unique. When user attempts to sign in through a provider in which his email is one that is already registered, user is redirected to the signin page with an error passed as a query string parameter.
Headers were a weird mixture of html5 and xhtml. Now it's pure html5.
Use this as doctype if you really want it to be xhtml, but I don't see why would you want that with Angular:
```html
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
```
