Is it possible to create email templates with CSS in Google App Engine Go?

I am creating a GAE Golang application that will be notifying users of what is happening in the system. Since I want the emails to look nice, I am already using HTMLBody. However, as I'm creating more and more complex emails, I would like to start using something like html/template to crease nice looking emails with CSS and so forth. However, I'm not sure how I can do Template.Execute to turn it into HTMLBody string that can be sent.

How can I use something like html/template to create HTML emails to use with appengine/mail ?

You can render the template to a temporary byte buffer, like this:

var tmpl = template.Must(template.ParseFiles("templates/email.html"))
buff := new(bytes.Buffer)
if err = tmpl.Execute(buff, struct{ Name string }{"Juliet"}); err != nil {
    panic(err.Error())
}
msg := &mail.Message{
    Sender:   "romeo@montague.com",
    To:       []string{"Juliet <juliet@capulet.org>"},
    Subject:  "See you tonight",
    Body:     "...you put here the non-HTML part...",
    HTMLBody: buff.String(),
}
c := appengine.NewContext(r)
if err := mail.Send(c, msg); err != nil {
    c.Errorf("Alas, my user, the email failed to sendeth: %v", err)
}