while specific extensions might be available on certain mailto handlers, the relevent text from RFC 2368 - The mailto URL scheme is
The "body" hname should contain the content for the first text/plain body part of the message.
The hname here refers to the 'body' in the 'body=some text' part of the mailto url. You basically get to provide a full set of email headers (name=value pairs) in the mailto url - separated by &
's - 'body' is defined specifically in the rfc to indicate the following text (up until the next separating '&') is the text/plain part of the message. Because there is no schema to add multiparts to an email message, you cannot express the multipart required to add the text/html part.
The only way to add a text/html part to the message would require you to be using a mailto handler that either supported an extended uri syntax, or had some way to allow you to continue programmatically adding content to the email.