What’s important to know here is the distinction between Article-type objects (meant to be formatted, long-form content; e.g. blog posts) and Note-type objects (meant to be short messages, e.g. tweets, Mastodon toots).

What’s also important to know is: Mastodon’s very concept is to specialise in being a classic, purist, minimalist, old-school, original-gangsta microblogging application. And Mastodon tends to stretch the ActivityPub spec until it breaks to live this concept.

Mastodon absolutely refuses to ever render more complex content in all its glory. For example, Mastodon refuses to show images embedded in content. Mastodon also refuses to support any lists except for bullet-point lists, and it sanitises e.g. numbered lists into bullet-point lists. Mastodon refuses to support horizontal rules. Etc.

Up until Mastodon 4.0, released in October, 2022 (right after Mastodon’s current culture had been coined), Mastodon flat-out refused to support any text formatting. Its HTML “sanitiser” would “sanitise” all content down to plain text.

Why? Because text formatting isn’t classic, purist, minimalist, old-school, original-gangsta microblogging.

What Mastodon is doing right now: It shows Note-type objects, but after “sanitising” them. As for Article-type objects, it renders them with their title (if there is any), their summary (if there is any; this is literally the first time ever that Mastodon acknowledges the summary field as a summary field) and a link to the original. Comments on this preview end up comments on the original. But nobody on Mastodon understands.

This came to pass around 2017/2018, after first Hubzilla and then Mastodon became the first two Fediverse applications to adopt ActivityPub.

Hubzilla, capable of both showing and generating the full set of HTML text formatting, mostly implemented the official W3C spec by the book. It sent its posts as Article-type objects and its comments as Note-type objects.

Mastodon, intentionally incapable of any text formatting, sent Note-type objects, but it “sanitised” the text formatting out of both Note-type objects and Article-type objects. It completely defaced especially the latter. Mastodon users wouldn’t know what the latter were supposed to look like because nobody ever looked them up at the source.

The Hubzilla side asked Gargron to stick to the official W3C ActivityPub spec, respect what Article-type objects are and implement full HTML rendering for them instead of defacing them. He refused.

The result was that Gargron showed Article-type objects as links to the original with the title above. The Hubzilla devs considered this censoring and silencing of a Mastodon competitor and resorted to switching from Article-type objects to Note-type objects until Mastodon supports full HTML rendering.

Fast-forward about seven or eight years or so. Actual long-form blogging with all shebang was growing in the Fediverse, partially powered by commercial actors (WordPress, Ghost, Flipboard). In a strange twist of faith, they didn’t “implement Mastodon” because they knew nothing about Mastodon’s peculiarities. Instead, they went by the book and sent Article-type objects.

They were quite surprised to find out that Mastodon linked to their posts instead of rendering them. I guess they were also surprised that Mastodon wouldn’t go render their post with all their formatting, even if it did render them. And they weren’t pleased. So another round of discussions started.

Result: Mastodon added the summary to the preview. Wow. Just wow.

GoToSocial, on the other hand, happily renders everything it receives as it is to my best knowledge. At least with an appropriate frontend that hasn’t forced all of Mastodon’s limitations upon itself, and that actually supports full HTML rendering even if Mastodon itself doesn’t.

Why? Because GoToSocial isn’t driven by that “everything on here must be classic, purist, minimalist, old-school, original-gangsta microblogging” mindset.

Mastodon has the same issue with Lemmy posts, it’s an issue with different activity types. I think Mastodon posts are “notes” and Lemmy/Phanpy posts are “articles”, so Mastodon won’t fully render them because they don’t really focus on compatibility with other platforms.