Squire

It is an HTML5 rich-text editor, which provides powerful cross-browser normalization in a flexible, lightweight package. It was designed to handle email composition for the FastMail web app. The most important consequence of this (and where Squire differs from most modern rich text editors) is that it must handle arbitrary HTML, because it may be used to forward or quote emails from third-parties and must be able to preserve their HTML without breaking the formatting.

Squire on GitHub