Why aren’t my preview cards being generated?

So, the way the “preview cards” work, is that the provider (facebook, twitter, Slack, etc) hit that URL with a special network request.

The application return to them a very small snippet of HTML, which they ‘sanitize’ to make sure it’s safe (of XSS, SQL-injection, etc). If they consider it safe, they display that snippet of HTML

A quick way to test it is to use https://cards-dev.twitter.com/validator