@joshix@renchap please don't. Mastodon is not designed for long posts and this is a bad hack.
Instead, we should just have a support for attaching markdown files to posts, which then would turn into embeds with image, title and description from the file. HTML is not allowed at all.
Clicking such an embed would open an article mode in supported clients, or, to maintain backwards compatibility, link to the special page that renders nothing but the article content and footer for your instance.