Squire is our rich text editor that powers the mail composer in Fastmail, providing support for formatting and WYSIWYG text editing in all modern browsers.
Now, there are a good number of rich text editors. However, most have the luxury of strongly limiting what the person can enter to ensure the data model doesn’t break. We can’t use those because we must be able to 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. This is Squire’s advantage.