TYPO3 Core

The "message-class" is determined from the severity inside render() when rendering HTML, which also renders a <div> around and can usually just be used to render a single message.

Extbase/fluid-issue #7098 brought up the idea of separating out a getClass()-functionality from the actual rendering. Since over there meanwhile t3lib_FlashMessage() is used, the idea is now to implement this part of the solution right there - in t3lib.