How to bold text in an email created with Javascript

Hello All, 

Need some assistance from the community in this. I am using Javascript to create an email with the users final score. That part I have working perfectly. What I am trying to do is have the score show as bold in the email body which is passed through as a variable. I have been scouring the internets for days and nothing seems to be doing the trick. Any suggestions?

Here is the working script:
var player = GetPlayer();

var email=player.GetVar("Email");

var subject="Score Results";

var OVERALLTOTAL=player.GetVar("OVERALLTOTAL");

var mailto_link='mailto:'+email+'?subject='+subject+'&body='+"Based on your responses , you scored a " +OVERALLTOTAL+"%0d%0A%0d%0ASave this email to track your results.";

win=window.open(mailto_link,'emailWin');