Pesticide outlines each element of an email to better see its placement on the page. I use this on a daily basis. When we get new email submissions, we use Pesticide to outline all the tables and images in the email and see if there are any problems with it.
Co-founder, Growth - Really Good Emails
1%Stacklist Startups Are Using Pesticide for Chrome
Pesticide is an open source CSS tool for debugging. The pesticide toggle outlines each element to better see placement on the page to more easily see what CSS is causing problems.