Atom, emacs, and vim have preview modes for markdown that include checkers. I like to use external editors because then it gives me an excuse to use git
to version control my posts.
Next evolution is to post to steemit directly from git
(See https://github.com/r351574nc3/docker-git-steem-bot)
VS Code, Sublime Text, and other GUI editors have the ability to use git directly in the editor.