myapp/app/views/layouts/mailer.html.erb
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<style>
/* Email styles need to be inline */
</style>
</head>
<body>
<div style="padding: 20px; font-family: sans-serif; line-height:22px;">
<%= yield %>
</div>
</body>
</html>