If you need to send an email, have you considered using E-Mail module with templates from the app store?
Adding some html tags is not possible https://stackoverflow.com/questions/5620324/mailto-link-with-html-body
For security reasons and compatibility it will probably not allowed. (Not all email client do support HTML bodies when this spec was written)
For mobile the Web Share API is use full, though it has no desktop support. Maybe you can combine it with the mail to link...