Mastodon Glitch Edition/markup
GlitchSoc has the ability to format posts with limited subsets of HTML or Markdown, which does not seem to be documented anywhere. (Update: this feature is apparently available now on trunk Mastodon. It's not clear whether it originated there or was backported from Glitch.)
It has to be enabled on a per-client (per-user) basis via
[Getting Started] →
[App Settings] →
[Compose box (tab)] -> check the "show content-type choice" checkbox. Once this is done, you should see a markup-selection icon between the "lock" icon and the "CW" icon, as seen below:
Note that apparently the only way to get
strikethrough in a post is to use Markdown, since neither <s> nor <strike> seem to work in HTML mode.
Also, it appears that HTML codes work in Markdown mode as well.