Rich content editor allowing HTML and Markdown

[Ambient-Impact]

The current editing experience on Omnipedia is fairly basic, with a giant plain text field. A rich text editor would help immensely, but it'll have to support our content formats, which are a mix of custom HTML elements and markdown.

CKEditor 5

This is now a stable Drupal core module as of Drupal 9.5 and has a fairly great UI that's also mobile optimized. It supports Markdown out of the box in the free version.

There's a handy (long) list of modules that have or are planning supporting CKEditor 5.

• Drupal core 10.1 supports the ckeditor5-autoformat plug-in, which makes editing Markdown even more pleasant.
• The CKEditor Markdown module seems to enable Markdown in CKEditor and supports Drupal 10.

Editor.md

Nice side-by-side UI; has stable release with security coverage, but no Drupal 10 support released, though it's in the dev version and Git.

Toast UI Editor

Looks interesting but the Drupal module is unsupported and the project is seeking new maintainers.