Email Best Practices: Plain Text vs Markdown vs HTML vs Rich Text

So, having compared HTML output from the various editors, it looks like the only meaningful difference is that Drafts encloses the external image links in p tags, while the others use figure. Figure seems overkill to me, but for whatever reason it appears to render better in Outlook. Is it possible to modify the HTML Preview action to close images with with figure tags? I guess I could take the html output and process it again with regex replacements. This is actually where I started because I’m using dropbox links for screenshots and needed to replace titles with ![] and dl=0 with raw=1. Sorry for the dumb questions. I’m not much of a coder. Any suggestions welcomed