The standard editor used to enter Stack Exchange posts, which supports a live preview of Markdown syntax.

The standard editor used to enter Stack Exchange questions, answers, wiki entries, user profile about me field, and so forth. It supports a live preview of Markdown syntax.

This editor is open source. And the Markdown is CommonMark compliant, with table support.

On the server it is Markdig:
https://github.com/xoofx/markdig

On the client it is markdown-it:
https://github.com/markdown-it/markdown-it