Sometimes all you want is a really simple responsive HTML email template. Here it is.
Preview: http://leemunroe.github.io/responsive-html-email-template/email.html
Use the template as is. They'll put the CSS inline for you when you put together your campaign.
For an API like Mailgun you need to put the CSS inline. You can use inliner tools like Putsmail or Juice to do this automatically.
- Copy all of email.html
- Paste the HTML as the source into Premailer
- Copy the HTML results and use them in your email view/template
Tested on mobile, desktop and web.