I have a couple of mailto links in a HTML document.
<a href="mailto:etc...">
Can I insert HTML formatted body in the mailto: part of the href?
<a href="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.
As you can see in RFC 6068, this is not possible at all:
The special
<hfname>"body" indicates that the associated<hfvalue>is the body of the message. The "body" field value is intended to contain the content for the first text/plain body part of the message. The "body" pseudo header field is primarily intended for the generation of short text messages for automatic processing (such as "subscribe" messages for mailing lists), not for general MIME bodies.