“At some stage in your career, it’s likely you’ll be asked by a client to design a HTML email. [...] Building an email is not like building for the web. While web browsers continue their onward march towards standards, many email clients have stubbornly stayed put. Some have even gone backwards.”
Rock Solid HTML Emails is a really useful article from David Greiner, co-founder of Campaign Monitor, aimed to help with HTML and CSS while creating rich e-mail content.
The article is part of the “24 ways to impress your friends” series, which is an advent calendar blog for web professionals. If you like the advent article idea, be sure to check out Performance Advent and PHP Advent too!