Its very common that your client asks to send the email to user which have good look and feel having the use of multiple font sizes, colors and images. Some time you may need to define the email template for the email which is shared by the client. This template is first converted in HTML and then added to the drupal backend to send the email to the user in this format having the dynamic values. So Adding the email template in drupal is the common requirement for all drupal developer and client may not ask for it at the time of the project scoping but will ask when doing the acceptation testing.
Drupal has the capability to do this for you without doing any coding. Use the two modules; one is Mail System and other one is Mime Mail.
Mail System
This module gives the option to control or to manage the mail sending functionality by drupal. There are two aspects in mail sending functionality; one is formatter which convert the massage to decided format(e.g. HTML, Plain text) and second one is sender which sends the formatted message to recipient. Here, Mail System module gives you the user interface to select the mail formatter and sender for mail system. Even you can decide which formatter and sender to be used for the specific module.
Mime Mail
This module helps other modules to send the HTML formatted message. It prepare the MIME-encoded data to be sent as HTML and convert the styles and graphics accordingly.
After Installing these both Modules, you can set the email formatting configurations as you may see in the following images.
Good explanation for these two modules.