What html/css attributes are mail safe?

Question

I have recently been developing a newsletter for a client of mine. How ever I can't seem to find good information on what css and html are safe to use in the major mail clients.

I thought that maybe there are people here that have the knowledge and we can create some sort of list of things that work in major mail clients.

This is a list of popular mail clients I borrowed from campaign monitor. (If I forgot somthing please tell me)

Microsoft Outlook
Apple Mail
Hotmail
Yahoo! Mail
Gmail

The question is what tags, attributes, special quirks are there in these major browsers and how can they be easily avoided.

Thanks for the help,

Answer 1

There is a detailed and comprehensive list of CSS support in common mail clients at Campaign Monitor.

http://www.campaignmonitor.com/css/

Answer 2

You can find a comprehensive list of supported and non-supported CSS features for all major email clients at Email Standard Project

A really useful bootstrap for developing HTML emails, which has a ton of discrepancy eliminators is HTML Email Bolerplate

And as a general rule - always use tables and all the old-school HTML tags ( align, center, valign, color etc. ). Some reading on the topic.