What are common issues with Outlook 2007/2010/2013?

Instead of using the IE-based HTML rendering engine, Outlook 2007/2010/2013 use the MS Word HTML rendering engine.  Microsoft Outlook is very unique as they are the only one to do this for their client.  Due to this, this can cause formatting issues quite commonly. Following things won't work in Outlook 2007/2010/2013: No background images in DIV tags and TABLE cells. No nested background colors. A background color in a DIV