I have a couple of mailto links in a HTML document.
<a href="https://stackoverflow.com/questions/5620324/mailto:etc...">
Can I insert HTML formatted body in the mailto: part of the href?
<a href="https://stackoverflow.com/questions/5620324/mailto:me@me.com?subject=Me&body=<b>ME</b>">Mail me</a>
Note that (2016) in iOS, it is perfectly fine to add <i> and <b> tags for simple italic, bold formatting.