When you want to start using Twitter cards you have to whitelist your domain through the Twitter cards validator. Once your domain is validated you are all set.

Note: when you change your cardtype, you have to revalidate.

• Visit the Twitter cards validator service.
• Enter a url which contains the Twitter cards meta tags. By default, the only suitable URL is a detail page of the Blog module.
• Fill out the form and submit.
• You will receive an e-mail when Twitter has completed your validation.

Once you have completed the steps above you will be able to use Twitter cards on your website. With Twitter cards you can give your followers a richer Twitter experience with fully detailed sharing of your pages.