This would be a handy link, but there are two important challenges.
First, you need to encode any reserved characters used in the URL. For example a line break must be encoded as "%0D%0A".
Second, you don't know which mail client will be used, so the rendering of HTML is unpredictable.
If implementing the functionality myself, I'd use a link that brings up an editor and/or sends the email from phpmail or a server side technology. You may have additional requirements.