MVP.css - Minimalist stylesheet for HTML elements
Design
Responsive HTML images
โฆ adapts your existing HTML images for mobile devices. No mark-up changes needed. Just drop it in and forget about it.
HTML E-Mail Boilerplate
This website and its sample code creates a template of sorts, absent of design or layout, that will help you avoid some of the major rendering problems with the most common email clients out there โ Gmail, Outlook, Yahoo Mail, etc. While not plug and play (you know, you'll have to do some work ;-), it will provide some helpful examples and snippets that will keep your email design rendering as true-to-form as possible.