I know it’s straying from the intended use of admonition blocks, but I found them a useful facility to create all sorts of semantically meaningful html constructs not easily doable with markdown alone, or not at all. Like figure elements with multiple images, or ML diagrams instead of images, etc. The .AlertTitle does understand markdown linebreaks (\end of line), since the character is not displayed unless escaped (\\). But they’re not applied: next line does not take part in .AlertTitle. Since markdown is already parsed and rendered, it would be nice if \actually worked. Sometimes titles do take multiple lines (I could see an ASCII art), or a figcaption might include a comment.
Hugo supports the Obsidian callout syntax which does not support hard line breaks.
But you can do this instead:
> [!NOTE] Radiation **hazard**<br>Please be careful.
> Useful information that users should know, even when skimming content.
For the above to work, you must do this in your project configuration:
[markup.goldmark.renderer]
unsafe = true
This is safe if you control the content.