“Markdeep Features: Admonitions”, Morgan McGuire (CSS, typography)⁠:

Admonitions are small break-out boxes [sometimes called ‘infoboxes’, associated with manicules] with notes, tips, warnings, etc. for the reader.

They begin with a title line of a pattern of three exclamation marks, an optional CSS class, and an optional title. All following lines that are indented at least three spaces are included in the body, which may include multiple paragraphs.

The default stylesheet provides classes for note (default), tip, warning, and error.