They can be done in HTML or CSS. I have seen them but in my opinion the cost of having them designed would far outweigh the rate of return. You could however sign up for services like Icontact where you can use templates, the do brochures, newsletters, etc. The only thing with those is if you get reports of spam they will terminate your account after so many. Your ISP could also do the same thing depending on how many you are sending out, etc.