Would be nice to be able to have the option 'Is HTML' in the 'send an approval email' so that there can be consistency between 'send an approval email' and also 'send an email' when both are required to be used within the same flow.
You can now include rich text in approvals. Techincally, it's markdown not HTML, but it lets you do the same scenarios: https://flow.microsoft.com/blog/rich-approvals-text-and-multiselect/
Markdown is half-baked. Please... PLEASE consider implementing a solution that allows HTML.