Flickr is an image and video hosting website.
Marfeel uses the iframe feature of Flickr to display an image from their website.
To implement the Flickr extension, Marfeel copies the iframe on the frontend along with its source.
On the backend, Marfeel inspects if the source of the iframe identified in an article is in the allowed sources. If it is, Marfeel displays the iframe on the frontend.
The iframe's source is only loaded when it's inside the viewport.