Confirmation email - providing a WYSIWYG so users can add HTML-ified content?
You can put basic HTML in the text box you have pictured above. Bold  works for email clients. The majority of email clients don't support every type of HTML content you see on the web. Web browsers are able to display scripts, animations, and complex navigation menus, while your typical email inbox isn't built to handle this type of content. 

Safe to use for most email clients:
static, table-based layouts

HTML tables and nested tables

template width of 600px-800px

simple, inline CSS

web-safe fonts