There have been many, many requests for HTML in approval emails. For some reason, this must be difficult to implement as it is not on the horizon. I find the markdown solution to be very unreliable and limiting. For example, when I embed links, they always break.
Could I suggest as a solution that an action be created that allows you to convert standard HTML to markdown? This would allow those of us already very familiar with html to design the layout and then have it be converted to markdown. I realise markdown does not have the range of flexibility in styling of html, but you could strip out anything that is not doable in markdown. There are converters available on line, so I would imagine this would not be too difficult and you already have an excellent action for parsing JSON. Something like that would be great.
In the longer term, I really think that HTML markup should be the way to go for approval emails, but this might be a good intermediary solution.