Summary cards are URL previews. Iframely generates it in case when no native embed codes are available or allowed in your API settings. We do it for articles, media players that are not whitelisted, and also for general URLs that have a thumbnail (thus, trying to avoid private links and internal in-app URLs).
When API suggests you a summary preview, the primary
rel array of Iframely API will contain
summary in it.
Example for articles
Here's a card for 500px ISO:
Example for general links
For links where we only have a smaller or portrait thumbnail, Iframely will generate a smaller card like this:
Change design of the cards
You can change the design for the cards via our flexible WYSIWYG editor. Load a few samples from presets to get some inspiration:
Let us know if you need any help with it.