Working with e-mail templates is quite different than working with websites, so you may find the following information of use in this chapter:
Customizing e-mail markup is a tricky business: the HTML used in e-mail templates needs to follow strict guidelines. You may find Campaign Monitor's resources at https://www.campaignmonitor.com/resources/will-it-work/.
Various e-mail clients will display the e-mails in various ways, much like different browsers can display the same website differently.
It is always recommended to retain as much as possible from Magento's default e-mail templates in order to make sure the mails are displayed correctly on as many clients as possible. This will also make the Magento upgrade progress much easier for you!
Bear in mind that most (if not all) e-mail programs don't display images by default. Be careful that your e-mails' core messages are contained within text in your e-mail and not in images!