Tragically, this also means that, even after 31 years, I’ve still never ‘get good’ enough.

Firstly, sorry for any potential derailment. This is a comment about the Markdown used in your post (I wouldn’t normally mention it, but consider it fair game since this is a ‘Fediverse’ community).
The spec for lemmy’s spoiler format is colon-colon-colon-space-spoiler. If you miss out the space, then whilst other Lemmy instances can reconstitute the Markdown to see this post as intended, Lemmy itself doesn’t generate the correct HTML when sending it out over ActivityPub. This means that other Fediverse apps that just look at the HTML (e.g. Mastodon, KBIN) can’t render it properly.
Screenshot from kbin:

Also, if you add a horizontal rule without a blank line above it, Markdown generally interprets this as meaning that you want the text above it to be a heading. So anything that doesn’t have the full force of Lemmy’s Markdown processor that is currently trying to re-make the HTML from Markdown now has to deal with the ending triple colons having ‘h2’ tags around it.
Screenshot from piefed:

(apologies again for being off-topic)

No, it’s still running. I just got out of the habit of adding posts. I didn’t realise it’d been 2 weeks. I’ll start it up again in a bit.

lemmy.world has the community, and is keeping up to date with it, because someone (me, actually) brought it through and subscribed.

midwest.social has the community, but won’t get further updates because no-one subscribed (so technically I don’t have the inbox address for it - it’ll be the same format as everyone else of course, but it would reject updates with the ‘community has no subscribers’ error if I sent stuff to it anyway).

justworks was in the same situation as midwest.social, but it has a subscriber now, so now it has everything and will keep up to date. I didn’t have to do anything - the new subscriber’s action re-fetched the outbox (discussed below) and luckily that contained everything that was missing. The post from infosec.exchange is there too, so that wasn’t affected by federation blocks.

lemmon.website isn’t running lemmy - the tails community is ‘virtual’ in that it’s just a bunch of static files pretending to be a real community. The main address 403s 'cos thats just a folder.

infosec.pub doesn’t have the community most likely because no-one who is logged in there has searched for it (instances won’t search outside their own database if the query isn’t from a logged-in user). lemmon.website is in their /instances list though, so there’s no blocking.

1. An outbox on lemmy contains the last 20 or so original Announces that the main community uses to tell the communities on other instances about a new post. To illustrate:
   curl --header 'accept: application/json' https://lemmy.world/c/microblogmemes/outbox | jq .orderedItems[0] would be for the most recent post on microblogmemes. (Op’s post is at orderedItems[6]).
   Fetching this allows a new instance to re-create recent posts, as if it had received them at the time.

2. Someone clicked !tails@lemmon.website, but didn’t go further (they probably got lemmy’s misleading error screen and gave up). As with midwest.social - no subscribers = no more updates.

3. Yeah, lemmon.website is mine. Not running lemmy means I fudge things a bit (including having posts from Mastodon users, of course)

Hmmm, that’s not defederation though. The community’s outbox has the 20 most recent posts in, and that cat gif is currently at number 20. The only way those earlier posts can be there are if someone brought though the community a day or so ago and then didn’t subscribe. If anyone ever does subscribe, I can manually send the rest (and can see if the infosec one fails)

Yeah, cross-posts would be a new post from me, rather than the original from e.g. George Takei, which isn’t ideal.

I hadn’t really thought about defederation between servers running both Mastodon and Lemmy. I guess that post wouldn’t show up on sh.itjust.works (I’ll have a look if anyone subscribes from there)

Screenshotting a post from another fediverse app seems a bit crazy. As an alternative, this post is available natively in lemmy, as text and from the original author (so you can reply to him if you’d like).

I can’t give a universal link to a post obviously, but if you’re on lemmy.world, it’s here: https://lemmy.world/post/11631169, and if you’re not, it’s available on the !tails@lemmon.website community.