I am interested in the concept of creating sites which are less harmful to the environment. The most damaging aspect of some designs is the size and amount of images on a site. Higher emissions of CO2 are produced by large image dominated sites, due to more loading and processing.
If you must have images on your site then reducing the original size and optimising them for the web is essential.
Choosing a web host which uses sustainable energy such as solar or wind power for hosting your site can be another step towards your green credentials.