mailto link with HTML body

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.

Whilst it is NOT possible to use HTML to format your email body you can add line breaks as has been previously suggested.

If you are able to use javascript then "encodeURIComponent()" might be of use like below...

var formattedBody = "FirstLine \n Second Line \n Third Line";
var mailToLink = "mailto:[email protected]?body=" + encodeURIComponent(formattedBody);
window.location.href = mailToLink;

No. This is not possible at all.