I thought this was just HTML, but sadly not so simple.
I found a useful guide here: https://sourceforge.net/p/dvdmetabase/discussion/markdown_syntax but it looks as though not all formatting is accepted in Metabase (for example bold using doesn’t work.
Is there a guide somewhere as to what’s supported?
Presume you are referring to Markdown in Text Cards introduced in version 0.28.0. Though a bit subtle there is a Markdown link to a guide on the syntax somewhere under the Adding headings or descriptions with text cards heading in the users guide.
I can confirm that bold using the **bold** MD syntax works-for-me 
Thanks for that.
One thing I’ve not been able to find - can you set a link to open a new window rather than leaving the dashboard?
Unfortunately that's not possible in Markdown, but you can of course ctrl- or command-click on a link to open it in a new tab/window with most browsers.