When DocC generates the archive and webpages, they seem to be missing. Then I tried to swap them with Markdown-syntax tables, and they display correctly.
As far as I can tell, DocC will strip most HTML tags (as well as their children), and for other tags children can be rendered as plain text without styling. I've written a few special rules to automatically transform common HTML tags into Markdown, including <table>.
Thanks for your answers!
It would be helpful if the DocC team can provide a list of supported syntax for Apple's documentation markup so there's less guessing game.
The current source of truth for this is here: Formatting your documentation content. But I agree it doesn't do a great job of fully specifying the syntax. @ronnqvist and @sofiaromorales have both been spending time recently thinking about how to improve the overall documentation story for Swift-DocC and that page has definitely come under scrutiny.