Can the petition message contain HTML?

Yes.

Since version 105.2.0 there is an HTML editor included as the default for editing the petition message. It works just like an WordPress text editor…because that’s what it is.

I have left the Markdown editor in place as a legacy for people who already have messages created with Markdown.

Note: if you switch from HTML to Markdown (not a good idea) or vice versa you may have to reformat your message, so choose wisely.