There are many editors you can use.

- Steemit ~~has~~ had HTML editor
- Busy has simple editor with shortcuts for markdown
- https://steemit.com/utopian-io/@tngflx/aren-t-you-tired-of-our-clunky-editor-introducing-steem-ultimate-editor

I believe you could find many other articles with list of Markdown editors. It may be even better for some to prepare the text somewhere else, copy it to the editor and do the final touches.

You can use almost any editor to write your posts but if you want to use the pull-left and pull-right classes, you will need to write them by hand (which is quite easy if you know where to put them) because they are specific to Steemit and Busy.

I personally like to see the original text with the markup with a previewer than to use any wysiwig editor.