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:x@y.com?body=" + encodeURIComponent(formattedBody);
window.location.href = mailToLink;