I have read that since various ESPs can be quite picky it is best to do anything you can with html so they can be more sure to render correctly. Hence transitional may be a good option so you can use bgcolor, etc. However I have also read that strict should be the norm and that any kind of styling should be left to css.

What is the best practice in this situation? Are there really any ESPs that will not render background color css if I use strict?