I don't see background-attachment:fixed in the Supported CSS table (http://www.campaignmonitor.com/css/). Why is that?
Hi ox4dboy, the 'background-attachment' property is both poorly supported and very rarely used, so we've decided not to include it in our Guide to CSS Support, as our chart is already quite full.
Thanks for your feedback! I've added your vote for this, and certainly if it becomes more supported and/or more commonly used in the future, we'll be happy to include it.
I use this quite a bit, it's not greatly support but because few people use it, it makes your emails stand out where it does work.
Where it's not supported it just falls back to scroll, so as long as you test that it looks ok in both states, go for it.