I wouldn’t expect two-line titles to be supported. A title should be one line by definition - it becomes the title of a browser tab, the name of a bookmark, and so on.
You might need to add your own field in the YAML meta and process it in the way that you want the “two-line” title to behave.
Can you provide an example of why you want a two-line title in the first place?
That indeed looks like something best resolved with b_rad’s subtitle idea.
Or, if all your titles need line breaks after the same character (like : in your example), you could probably use some frontend trick to make them display so. But this is just a workaround.
Yes, the outline algorithm hasn’t been implemented by any browsers yet, but a couple things to note.
the following isn’t really accessible
If it is properly implemented in the underlying technology, it should make the content more accessible to let the user know that the title is grouped with subheadings, subtitles or taglines. I believe that is one of the primary reasons for hgroup. Additionally, using a p element will look much different in a text browser.
the use of <hgroup> is debatable
Not as much as you may think. As you can see in the link you sent, the date of the HTML 5.2 spec is 2017-12-14. W3C no longer maintains the HTML spec as they reached an agreement with WHATWG in 2019. WHATWG is the official HTML spec as you can see by clicking on the link under Latest published version of HTML here:
WHATWG still includes hgroup and uses it in many examples, and therefore it is part of the official spec. The information on MDN appears to simply be either outdated, or questionable regarding hgroup, but either way you can still use their suggestions just fine as you did in your example.