For quite some time I’ve been trying, and until just now failing, to get Mastodon to display preview images for my blog posts. It seemed to me that all I had to do was add the proper OpenGraph og:type, og:title, and og:image tags, right? Right?

And then if I check it with one of the numerous preview checkers, and I see the image, it ought to work, right? Right?

No! It doesn’t work that way. This page is a test to see if I was missing something in particular. Maybe it was the lack of og:url (which is required given the spec)? Maybe og:description? Maybe og:image:alt?

And the winner is: og:url, which isn’t that surprising since as the spec says:

og:url - The canonical URL of your object that will be used as its permanent ID in the graph, e.g., “https://www.imdb.com/title/tt0117500/".

But you should add og:image:alt and og:description too, as it is the kind and loving thing to do.